The actual tops as a whole are pretty cheap wherever you look. The quality typically is a "you get what you pay for" but it should be $75-$100 variance unless you go with a cloth top.
Also, you typically need to get the rear curtain that holds the rear window seperately. They come in the clear plastic and glass. Glass is obvioulsy more.
The big expense with top replacement is really in the install price. Last time I paid someone was 5 years ago. I bought the top and had the installer put it in and it cost me around $500 to get it installed (I am sure there are varying prices but he was really close to the house). I think the top and rear curtain with glass window was around $200.
never had any experience with the ebay vendor's tops but you never know.