If you've blown a midrange driver, you could always consider having that driver replaced. If you'd prefer to replace the JBL ND310 speakers entirely, your price constraint isn't unrealistic. See an example here:

http://www.audioadvisor.com/prodinfo.asp?number=ENC500

Also...buying used speakers could bring you some good bang for your buck. And if you want JBL, you might check eBay. The Harman group sells there...JBL direct from the manufacturer.