We have all versions of minecraft - ipad/iphone app, xbox and computer. My DS and all of his similar aged friends are so into this game, it's a little crazy! They all prefer to play on the computer, but if they must, they'll pull out ipads/iphones/ipods and play that way.
anyway, to answer your question....Target has minecraft gift cards that are the cost of the online/computer version for $27. Just saw them last night. That will allow them to download the game to their computer and play away. There is no montly fee for this game - just the one time charge - which is really wonderful.