Hmmmm, best and cheap are rarely used together in the same sentence for pretty much everything. Also, what do you mean by "best grass"? Darkest green? Fastest germination? Aggressive spreading? Drought tolerance? Disease resistance? A combination of those attributes? Being in the Philadelphia suburbs you should stick with the cool season grasses, Kentucky Bluegrass (KBG), Turf type Tall Fescue (TTTF) and Perennial Ryegrass (PR). There are a few others but they are specialty grasses that generally aren't used for home lawns unless certain conditions apply. Also, do you know what kind of grass you have now? If you are trying to fill in bare areas with a type of grass that is markedly different than what you have, then those areas will stand out. The best strategy might be overseeding the whole lawn with whatever grass you decide to use, and go heavier on the bare areas. In terms of cost, most grass seeds are similar in cost per square foot, but some a lot of the cheaper bags you find at the big box stores contain mediocre or inferior cultivars that have been around for 20 years. Also you can find weed seeds in those cheaper bags, so saving something like 20 bucks really isn't worth it. As far as which grass to use, well that's a question we could spend a lot of time on discussing. I will try to outline some qualities of each type. PR: Easiest to grow, germinates in a few days (3-6 is typical), establishes fast, most newer cultivars have dark green color with fine blades. It is relatively drought tolerant, but will die if it isn't watered in the conditions we saw last summer. It's a bunch type grass that doesn't spread (although two new spreading varieties have just been released for sale), and usually requires periodic overseeding every few years. Not the most disease resistent in hot humid weather. Pretty wear tolerant. Not very shade tolerant, will look great for a year in shade but then will thin out. TTTF: Germinates slower than PR, but still relatively quick (6-14 days but usually 5-8 days in ideal conditions). Ready for first mow in about a month. Most varieties are medium to dark green, blade width is variable but generally not as fine as PR, medium to wide depending on cultivar. Very drought tolerant but only when mature and if soil allows deep rooting. Can go dormant for a short time but extended drought will kill it. Disease resistence is good in certain cultivars but can be susceptible to brown patch. Good wear tolerance, but also a bunch grass, with limited spreading ability despite the claims of newer spreading cultivars. Most spreading is seen years after initial seeding, and periodic overseedings are often necessary. Best of the three in terms of shade tolerance (can get by with 3 hours of sun a day). KBG: Germination is highly variable but generally takes 2 to 3 weeks except for a few cultivars that are faster (5-10 days). Most are slow to establish, with exceptions. Color ranges from medium green to dark green, with some being dark blue green. Blade width variable, usually medium to fine. Drought tolerance is lower than PR or TTTF, but has the ability to go dormant during drought, survive, and reanimate with regular waterings. Good wear tolerance plus has spreading ability and can fill in bare areas about the size of a large dinner plate in one year. Can have some disease problems, but not as susceptible to brown patch. Shade tolerance is low, not as low as PR, and some cultivars have better shade tolerance than others. Powdery mildew can be a problem in the shade. I hope this helps, and feel free to ask any more questions you might have.

OK so where can I buy a decent and cheap strain of TTTF?

Sorry make that KGB

So you want to try KBG. How much do you need? KBG is generally seeded at 2-3 lbs per thousand square feet for new lawns, 1-1.5 lbs per 1000 for overseeding. Again, I don't advocate spending the least amount possible on grass seed but if you choose to do so there are some blends at the big box stores that are OK, Pennington's carries a KBG mix that isn't too bad, Scott's KBG is OK too. They use older cultivars that perform well, but definitely not as nice as newer ones, but I'm assuming you aren't after an award winning lawn. Just make sure that there is 0.0% weed seed listed on the tag. Stay away from Vigoro brand, theirs' usually contains Annual Bluegrass, which you don't want. If you need a lot of seed Lesco can hook you up with 50 lbs for a reasonable price. Keep in mind, almost every KBG you buy at a local store will take 2-3 weeks to germinate, and that means watering the area lightly 2-3 times a day. If those seeds dry out, then you won't see much new grass. If you want to get a quick germinating variety, then you will have to use the internet, unless you have a good seed store nearby that carries a lot of KBG, but even then it's no guarantee. If you are planning on seeding this spring bear in mind that the best time to seed KBG is actually in late summer. Spring seedings usually don't do as well due to the pressure from summer heat and humidity, as well as weed pressure.

Where can I buy a quick germinating variety for a decent price online?

You've got competing and mutually exclusive requirements. KBG is not a quick germinating variety. It takes 3 weeks or so to germinate. If you buy a mixture of KBG, PR and TTTF, the PR will germinate in a few days and the TTTF in about a week or so. Many people stop watering at that point and the KBG never germinates. Since both PR and TTTF are bunch grasses, bare spots that develop don't fill in (or they fill in with weeds).

BP, while almost all KBG takes 2-3 weeks to germinate, there are two varieties that I know of that are much shorter. Orfeo is one such variety. Under ideal conditions Orfeo germinates in 5-7 days. http://www.sroseed.com/Products/PDF/Orfeo_ts.pdf .
I recommended this variety last fall on a friends lawn and it came up at the same time as TTTF. Another cultivar is Barimpala. This one germinates fairly quickly under ideal conditons (soil temps above 68 degrees) usually about 65% germination after 8 days. See page six of this pdf:
http://www.barenbrug.com/documents/1001/turf_guide_2010-final.pdf
and here's the pdf for the actual grass:
http://www.toddvalleyfarms.com/PDF%20Files/Barimpala.pdf .
Now if you plant these seeds in spring when soil temps are below 60 degrees I would bet that germination times will be longer than advertised, but still shorter than the majority of cultivars. You can get Orfeo at http://www.pawneebuttesseed.com/ . Here is one place you can get Barimpala
http://www.starnursery.com/seeds-accessories/grass-seed/wonderlawn-kentucky-bluegrass-seed-3-lb.html