The last time I got one I think the cost was around 18.00(I would plan on 20.00).
You can get Mr.Potato Head from some of the hotels(Pop century for one) and I know for sure you can get them in Dinoland at AK(can't remember the store name).
As to how many parts can you fit in? That is hard to say,there are some really big pieces,lots of medium pieces,and a few small pieces. It really depends on what pieces you choose to pick.
To get some extra pieces in to your box don't forget to fill up Mr. Potato's compartment in the back.
I would see if I could purchase the really big pieces like Dumbo apart from the box.
Just for your info, Mr. Potato head has both right hands and left hands-they are not interchangable. I can't tell you how many people on this board,me included,that have found this out the hard way.