My personal experience with fish finders is that you really can't go wrong with one of the Lowrance models. Since you already have the GPS you have some pretty good choices within your price range, from the X510C (Color) to X96 (Black & White). I've owned Lowrance products and have always been extremely happy with their performance, especially when you become familiar with getting the most out of the unit. I fished with people who have had other manufacturers and models and there's really nothing wrong with them but, in my opinion, Lowrance is truly second to none when it comes to fish finders. I would encourage you to take a look at this link, Lowrance has both tutorials and guides about selecting sonars. Depending on what you intend to do with the unit this little bit of reading will probably go along way to making sure you are happy with your investment.
Good Luck.