Asking the price of something bought: How much did you buy it for? For how much did you buy it? (grammatically ok, less common) How much did you pay for it? How much did it cost? What did it cost?

Buy/ bought refers to the item. Pay/paid refers to money. I bought a sandwich. I paid $4.99 plus tax.

The first would have to be "How much did you buy?" It does not mean the same thing. It means "How many items did you buy?" Buy and pay have slightly different meanings.

How much did you buy it? doesn't make sense unless you mean how much of it you did you buy? the other one makes sense