I am not sure if this is what you are after but try setting up an account on
redbubble.com, then buy your own stuff and see if it is as cheap as you want. You could try imagekind.com to do the same thing.
Also, you could use
Smugmug or
Ritzpix. Either one can print out poster sized prints, though they may be more expensive than you want.
Depending on where you live the cheapest thing might be to just run down to a copy store and see if they have any scraps or odds and ends poster paper that they would print what you want printed on for a discount.
Hi I have just bought an Epson 3000 A2 printer, and am looking to start up a small buisness doing just that.
I still need to buy paper but that can soon be done. If you can email me and let me know what you want , what for, what you can afford, and where you are. I may be able to help.
Chris