Got to say I agree with what's been said, if you flatten it out for a few days to lessen the folds, and them frame it, it will look great as is.
Save your money.
If how ever you still want it backed, I have some posters that are now finished being backed, and restored at Graham Bignells, I just need to get down there and collect them. I'll post them to the board when I get them up here, and you can see if he's worth giving a go to.