How much does a minecraft prepaid card cost?

Exactly what the game costs; $26.95 in the US.

Within the US…

A "Standard" prepaid card (for 1 account license) will run you $27 US / € 24 / £18… Which is the standard price online.

A "Two pack" prepaid card (for 2 account licenses… One for yourself & another for a friend) will run you $50 US (unknown for Europe, England & elsewhere). There's a small discount ($4 US total) as you're buying multiples.

Please be aware that prices quoted are to the next whole currency mark & sales tax / VAT (where legally applicable) is NOT included.