exhaust leaks (or bad mufflers) can cause worse fuel mileage, depending on the car. Some cars require more back pressure to run smoothly. I don't think it will be a big loss, prob like a mile per gallon or so.
If you bring it in to a muffler shop should cost you around $100 - $150 depending on how bad the exhaust system is. If it is just the muffler it might be cheaper. If the cadillac converter is bad it will cost you more than double that.
As far as getting better mileage, just keep your car up to date on all normal services, like oil changes and air filters and tire pressure. Also slower starts from a stop will help and slowing down.
Hope this helps