First of all, RELAX. If your trip isn't until late September ( 77 days from July 10th ? ) then you won't have any trouble finding transportation to Prague and back for a lot less than 450 E for two.

On the other hand if your trip is next week you may be in trouble.

Spend some time surfing the Eurolines and Bahn.de websites. They will both speak to you in English, well mostly, although it doesn't hurt to learn a bit of German.

Eurolines has 30 day advance tickets from Frankfurt to Prague for 35E one way, 62E return.

And on the Bahn.de website should be able to get tickets on the CNL 459 ( City Night Line ) train for 33 E one-way. This would be my choice for your trip. The CNL 459 departs Frankfurt Sudbanhof shortly after midnight, and arrives in Praha at around 11 A.M. No change of train, and you save the cost of a nights accomodation. On the way back you want the CNL458 which departs Praha at 6:30 P.M., and arrives back in Frankfurt at 4 A.M.

Enjoy your trip. Prague is a great city. But there is lots to see and do in and around Frankfurt as well.