Newb question, but why's it expensive, aren't they mining the whole time and can therefore make the usual money from that mining?

AFAIK Qubic (the company) is paying people extra to mine XMR through Qubic (if you mine $1 worth of XMR, you get $1.50 worth of QUBIC (the coin) which you can then sell). Qubic (indirectly) loses those extra $0.50. If on average the miners sell too much (more than two thirds of the rewards), then Qubic has to buy their own coins back in order to keep the price stable. Qubic bets on their coin pumping from the publicity.

You are correct. It's expensive if you want to go rewrite history. 51% is when that becomes economically viable to do on its own.