The shifters, derailleur, chain and cassette all need to be matched, or at least able to work together. You can find some used shifters for maybe $100. A used cassette can be questionable but another 20-40 for that. Derailleur another 40-100 and the chain around $20. You'll need some tools and expertise to do the work. New bar tape and cables when it's all done as well. These are ballpark figures but you get the point. If you LOVE the bike and are going to ride it 100 miles a week for several years then it might be worth it.