The value of old costume jewelry is all over the place. You can find plastic bags full of it at thrift stores, and you can see it for sale at online auctions for some hefty prices. It all depends on what you have, how old it is, how desirable it is today, and the condition.
Before you sell your old jewelry, you should learn more about it so you can estimate their value. There are so many books on the subject; I am sure that you can find some at your local library.