Ask friends. If there's a repair shop with a good local reputation, bring it to them and ask them what it would cost to replace what's broken. (Some shops will charge less if it's just the gllass/digitizer, although it's a lot more labor than replacing the whole screen, which costs more for the part - so there's no way anyone but the shop that's going to do the repair can estimate the price.) If it costs more to fix than to buy a new phone (there are some phones that sell for under $50 new), it's cheaper to buy a new one. If the repair is a lot cheaper (saving $20 to get a repaired used phone isn't worth it - keep the old one for parts and a spare battery) have it fixed.