Can cloth seats with airbags be reupholstered with leather? How much should I expect to pay for a good job?

It would be worth it to buy some upholstery cleaner and clean the seats really well, or have them professionally cleaned and then sprayed with Scotch Guard.

But if you do want to have them reupholstered, you need to have a professional do the job, preferably by an authorized Mazda dealer, because misplacement of the seat covers, even by a fraction, can cause the airbags to fail in an accident. So then, you'd not only be upset because you'd killed several cows to make car seat covers that didn't work, you'd be dead (maybe).

It may be cheaper to buy the seats off ebay or from a salvage yard and replace. To see if yours can be reupholstered, you must take it to a shop that does that.