how would this be for a cage?

That auction is only for the floor mat, not for the play pen itself. The walls of those type of play pens tend to be shorter than the grids we make cages out of, so you probably wouldn't want to use it for an actual cage. Outdoor should be fine though. It would most likely be a ton cheaper for you to buy an extra box of grids and a package of cable ties and tie them together to make a fence to put in your yard.

I agree with Percy's mom. Just a box of grids will make you a nice run with a top on it too!