I live fairly close to there and have been there heaps in recent months. It's a great place to go to in early spring, especially, before it really heats up. You can get really nice sized, healthy roses for $20, that look like they've been in the pot since the previous year. They don't have much shelter there, so the pots can get knocked around a bit when it's extremely hot and they were badly affected by the large amounts of rain we had a month or two ago (so the plants they had left got a lot of blackspot etc), but the new stock looks significantly better. They ended up offloading most of the damaged stuff for about $11 each. If you don't mind waiting a year or so, they would be very good value.
I also had to be very disciplined not to buy much last time I went in (about two weeks ago), otherwise I could have very easily have walked out with about 6 plants! (the newer ones were $25 each, the older stock that was still in excellent condition was $20)
They're also really good to deal with and use extremely expensive potting mix when they pot up their bare rooted roses.
For those on the other side of town, there's "A Rose Paradise", in Shannon St, Box Hill. They have older, more established bushes, that are more likely to be 3-4 years old. They're more expensive, though, at around $34/$35 each, in 10 inch pots. If you want a bush that will give you lots of flowers first up, they're a good place to go.
Just be careful to not be seen stepping on the rose bed in the front courtyard, though, or you could get an earful, like I did when I put one foot on it (lightly), to smell a rose. I was asked not to "trample" on the bed, as though I'd been jumping up and down on it.
For the best overall package of quality, plant size, variety, price and ease of viewing, the best I've come across is Newstead Rose Nursery, in Monbulk Rd, Monbulk (just down from Tesselars Tulip Farm). When I first walked in there I was really excited by the look of the plants (and how many they had) and was then even more blown away when I saw the prices. I was expecting them to be $30 each, maybe more, but I think the most expensive bush roses were the Delbards, at $22.50 each. And they were an excellent size and healthy looking. The "stock standard/common" roses, like Blue Moon, Camp David, Charles De Gaulle, Mr Lincoln, Paradise, etc, were just $15, for very good sized and extremely healthy bushes.
I've seen other places with roses for $30 that weren't a third the size of these ones, or as healthy. The standard roses they had were also extremely good value. I so wished I'd found that place before I bought so many inferior bushes, at more expensive prices, elsewhere. Probably the only downside was that they didn't seem to have many/any "exotic"/hard to get varieties. I'm down to a shortlist of about 4 types that I really want badly (Osiria, Beauty Star, Joy Of Life and Toro and maybe Purple Tiger) and they didn't have any of them, so I unfortunately didn't buy any,...

Just went back to 'The Rose Paradise' in Box Hill on Sunday (if you weren't aware it was there, you'd probably drive straight past it, as it's in a suburban street, with little advertising). Man their roses are big! I had a little more time to look around than last time and I'm about 6 foot one and a half (186cm) and some of their regular bush roses for sale are seriously almost as tall as me. They're in 10 inch wide pots, but they're only about an inch or two taller than the pots you see most roses in, at local nurseries, or places like Bunnings/KMart, etc. Yet the roses for sale at places like that are often below my knee height, yet may be only $4.50 cheaper! It's laughable. Some at Paradise had growth up to my chin and some were probably even over my head. And we're talking about very highly sought after varieties. Some would be 4 years old. Whoa man, if I had more free space and a whole lot of spare money I would have gone nuts (although getting them in the car in one go would be a big issue).
The bloke that runs it is also very knowledgeable and I learned a lot, like how much to prune a bush at this time of year and when it's best to cut roses for a vase (very early morning, or after dark).
I found one variety that I've been extremely keen to get, since I saw some in a vase, at a rose show recently (Joy Of Life) and it had just finished blooming, so he asked if I'd like him to prune it. I said OK and he pruned the hell out of it! Left basically only the two main, thick canes and the minimal amount of brand-new growth that was coming out of them (he cut off all others that were thinner than a pencil). So I walked out past all these tall, thick, luscious plants, some with buds/flowers galore on them, with my very-bare-looking-in-comparison one, but at least I got one I really wanted. He reckons that it will be in full bloom again in 6 weeks, if I take good care of it and next year will hopefully be really big. It would be probably 3 years old.
The only thing is that it won't be as much fun going to local nurseries etc any more, as every time I see a rose of interest at one of them, I'll think of how they're terrible value in comparison to those at Rose Paradise (or the rose nursery in Monbulk). Small price to pay, though.
About the only minor negative about the place is that they often only have very small tags on the roses (which are sometimes hard to find, with all that growth), so unless you know exactly what you want and ask them where it is, it can take a fair while to see everything they've got.