Farmville in its current state is pretty extensive with features. It would probably cost on the order of $300k. Add $100k if you want to make a game that's not mostly a straight rip off. Mafia wars probably a lot less like $100k last I looked at it. That's if you want to get it developed by people who know what they're doing. If you want to piece it together from indians and freelancers probably less but it would probably never get done. Those are Facebook game estimates mobile would be a bit less.
Anyway that's also only the development costs you'll need probably another $200k to have a shot at marketing it to the profitability point possibly less if you paid your dues with design and made a game that's decently novel.