Ok, let me mull over this tomorrow and play with it. As for the cost of the board, how quickly do you want it done? If you don't mind waiting, you can get ten (10) 50mm x 50mm boards for $9.90 + s/h. It just takes upwards of two weeks to get.
Personally, I prefer not to have breakout boards hanging off of the edges, I'd rather place them over an open area on the main board for support. But we'll see what I can come up with. The majority of the space will be taken up by the Mini as it is.
Another question, you want everything to be through hole, right (since you mentioned you already have it working on breadboard, I'm assuming you're just going to transfer from one to the other)?