If your AC unit is 30 years old it's probably has an 8 SEER rating. Purchasing a new system,even with the present minimum of 13 SEER,should in theory result in a 38% reduction in your electric bill.

I think you need to have your duct work evaluated as most likely it's the original and may have problems contributing to your high electric bills.

I don't think you'll get much help with an attic fan (some studies show they actually suck air conditioned air from the rooms below). Therefore, sounds like it's the 400 or so square feet of the converted carport that is your major concern. If there's attic space above the carport than insulate it. If not I can't think of any easy/inexpensive way to insulate it.

Adding insulation to an attic is the most cost effective improvment. I like batt insulation, but you may need to use the blown in type if you have a very constricted area.

Powered attic fans can cause more harm then good if the attic is not properly vented. A negative pressure condition can be created if the soffit vents cannot supply enough air to keep up with the fan. This could cause conditioned air to be sucked up into the attic. You may be better off with a passive turbine vent.

I am impressed your AC has lasted 30 years in the Florida summers. I know you have have spent money repairing it, but consider replacing it soon. You could cut your AC power bill in half with a more efficient unit.

Blown in insulation works great. I've also done fiberglass batts. Insulating an attic can be a DIY if your carefull not to fall thru your ceiling. You might save alot of energy with a new unit and it might pay for itself in a reasonable amount of time. I like highest efficiency single stage units of 15 seer myself.

Attic ventilation fans are generally a bad idea. They use energy and if not carefully balanced, they can suck conditioned air out of the house through perforations (leaks) between the living space and attic.

The rules for sealing your house and insulating are different in the humid South than in cooler and drier parts of the country. Some good places to look for advice are probably land grand universities in that area, University of Florida, Louisiana State University,�. In addition, http://www.buildingscience.com/ has a lot of good information for hot, humid climate building. Last, but certainly not least, consider hiring an energy rater. Do a little research first. Interview more than one on the phone, but asking them what in general they would do with a home like yours and if they know of any local rebate programs that would pay for improvements or their services. http://www.resnet.us/trade/home-energy-raters-hers-raters. They will be able to tell you a lot more when they are in the house and do testing.

To summarize, when homes were not mechanically cooled, a vented attic made some sense. Now, with older-style homes that have added AC ducts in the attic, it makes more sense to seal the attic and spray foam the underside of the roof deck with low density foam insulation. You no longer have to deal with all the leaks between the house and the attic and your ducts will be cool and the leaks in them less relevant. Your ducts are now outside the envelope of your house so when the leak your are blowing air outside. That air is replaced through leaks in your rooms. Beyond being wasteful, it can cause condensation in places where you don�t notice it and you get mold growth.

Lastly, if you want efficient AC and want to get rid of those pesky ducts to get rid of their pressure differentials forever consider installing heat pump mini splits. You will have effective zoning with them too and that can save you a bundle depending on your lifestyle and the layout of your home.

someone needs to get into your attic and look around.
the best scenerio would be to hire someone who can advise
you as to your best options for your specific home.
there are many energy raters in florida, you could
make some calls and hire an independent third party.
I don't know how it works with BPI, but as a Resnet
energy rater we do inspection & testing for air leakage into the house, and duct leakage.
there are two seperate
testing dates, prior to any upgrade, and once upgrades
are completed. the first to establish a baseline for
your house, and advise on upgrades. the last is to
verify that house leakage, duct leakage and any issues
that were addressed accomplished what they were bid to do.

as your house has both cinderblock and stick frame construction, and age of house I would suggest that you have considerable leakage. blower door & duct testing would
measure and pinpoint the leakage areas, so that they
can be addressed.
a couple of cases of caulk can do wonders to reducing
air leakge into the house. as can mastic sealing of ductwork.

trying to heat and cool a leaky house is expensive.
if you make the house tighter and reduce the duct
leakage it is a good investment.
when replacing a unit as old as yours with a more
efficient unit could cause problems for ductwork
in a vented attic. your existing ductwork's r-value
is somewhere between R-2 & R-4. with new variable
speed equipment the low speed of the unit will be
colder than high speed. ductwork in a vented attic
in our hot humid climate condensates.
installing open cell foam to fill the roof rafters
to full depth, and to cover the faces of the rafters
is a good investment.
this would make mastic sealing of ductwork viable
as compared to upgrading ductwork to R-8.
it would also put ductwork in a conditioned space
as the attic is now unvented.

if you can get a foam company that has worked with
hvac contractors..or visaversa..you will be dealing
with like minded people. it makes a difference.

most of our house leakage is from the living to the
attic space. moving the thermal (insulation) barrier
to the roofline with foam also moves the air barrier
to the roofline. so the recessed lights don't suck
air from the attic.

cost of foam is higher, and you should keep in mind that
you still need to meet code. this is not the 3-4" average
fill bid. R-values for La. are R-30 for attics
we meet the cathedral attic code with 6-7"
get several bids and ask to visit an ongoing or previous install.
understand that while the depth is increased that once they are on the job and set up its a matter of spraying more product. it doesn't double the price.

as to where it leaks in the walls of the house, the
blower door test will pinpoint these areas for you.
location of the leaks depend on construction of house.
each house is different.

best of luck.