Not a direct answer to the question you asked, but I have been able to do partial mashes and mini all grain mashes with a 10 quart (2.5 gallon) stockpot, then use my big pot for sparging and boiling. A smaller stockpot like that is really cheap.
Edit: Also, part of the purpose of the sparge is to rinse the sugar off of the grains with fresh water. Using the mash water as sparge water would "probably" leave a lot of the sugars stuck to the grains, and make your extraction efficiency pretty low. I have heard of "no sparge" brewing, but am too new and unknowledgable to comment on that type of brewing