As with all precious and semi-precious stones, the value of jasper depends on its size, cut, and quality. You'll need to take your stone to a licensed valuer or jeweler to have it valued.
You need to read this site, it cautions you about fake Jade:
http://reviews.ebay.com/WYOMING-NEPHRITE-JADE-MARKET-OF-TODAY-buyer-beware_W0QQugidZ10000000002756872