You need to contact an architect/contractor. Too many variables to even make a wild guess.

It depends on square footage, level of finishes,what part of the country you are in, etc. Best suggestion is to contact some general contractors/builders in your area and get rough estimates from them

First thing to do is check with local codes to see if this can even be done. There are often local regulations around converting garages and adding square footage. It will likely be cheaper to add on a living room than converting the garage and then adding on a garage. Garage conversions are some of the most expensive.

really? you want us to give you a price with that smidgeon of info you've given us? no square footage, no idea where you're located, what type of house you have, if you're in the city or in the boonies, if your current roofline will allow the add-on, or even if code will allow this, etc.

seriously, get an architect and have some plans drawn up.

Jury's still out on that, but it's possibly, probably, perhaps, perchance going to be more feasible to keep what you have and add on a living room.