Plymouth Neon hesitating, backfiring on acceleration

Hello. My friend had a tune up done on his 2000 Plymouth Neon and it still has little power or hesitates during acceleration. It also backfires when he hits the gas in drive but not in park. The mechanic claims it is his distributor module and he says he can fix it for about 300 dollars. My question is what could be causing this problem and, if it is the module, should it really cost 300 to fix? Any info would be helpful. Thanx.