I was actually in the very same predicament up until about a week ago. I'm buying a flemish giant towards the end of March,and findinga sutable cage for a decent price presented a challenge, but after some brainstorming and hours of Google research, i found these
http://www.allpetfurniture.com/Midwest-Pets-54-X-MDW1097.html
this is a dog play pen with a walk in door. Each panel is 24'' wide, so it can be shaped into a 4'X4' square, plenty of room for a bed, litter box and room to play! it's what i have decided to buy for my new friend
but if you're not planning on getting a bunny as big as mine, this may work for you also
http://www.petstore.com/ps_viewitem.aspx?idproduct=IL62237&child=IL62237&utm_source=nextag&utm_medium=cse&utm_campaign=nextag&utm_content=IL62237
it comes with a mat to place underneath...it doesnt include the handy door feature that the other does, but is still a great deal
i would also suggest Ebay to find cheap pens, but only with the warning that the dog pens starting at 99 cents sound like a fantastic deal, but often go for far more than what they are worth by the end of the auction, and also look out for "AMAZING DEALS" for cages selling for 1.99$ etc, but in fine print have 50-60$ shipping...
good luck, and i hope i helped!!
-Audrey