ThinkGeek offers the Iron Man Mr. Potato Head for $13.99. Coupon code "GEMINIO" yields free shipping. (Amazon charges pennies more for Prime members.) That's the lowest total price we could find by $7. It features removable armor and body parts.
Also available is the Thor Mr. Potato Head for $13.99 with free shipping via the above coupon code. That's another price low, this time by $2. You should probably get both and mix up their pieces, because oh man, how annoyed would Tony Stark be if you made him wear Thor's helmet?