Prices vary all over the board, but it shouldn't be expensive.

Given the bike, your best be might be a bike co-op, where they'll do it at low cost, or better yet help you do it yourself. Here's a link to a list of bicycle co-ops throughout the USA, but it's not complete by any standards. Search for you in your community under Bicycle co-op, community bike shop, or bike kitchen.

BTW- if you list your city of residence in your profile, someone might recommend a decent bike shop.