Yaffle,
I found some 'Gorilla Braid' fishing line being blown out on sale at WalMart, I think it was a 600 yard spool for around $9 a few years back. It's a gel spun line, not as 'tight' (thin)as the 'premium' fishing braids, but great for backing.
What I got was 50 pound test, and it was about the diameter of 25# dacron. Wish I had bought more of it.
There are some slight disadvantages to using super braids like this for backing. It's very 'slick' so you have to attach it to the spool properly or it will spin. It can bury into itself, so you have to spool it properly, and TIGHTLY. You also don't grab this stuff with your hands if it's under tension. It can and will cut you (I'd never do this with any backing ,so I don't worry about it).
The advantages, though, are why I prefer it. It's impervious to ambiant temperature, ultraviolet light, and moisture. It floats. It won't ever rot. It's lighter overall and lighter in the same 'bulk' (a reel 'filled' with it will be lighter than one filled with dacron, even if there is twice as much of it in 'length').
Another 'source' for long lengths of this kind of line that may be less expensive is a kite store. They use it as string for the really high flying kites. It's lighter, stronger, and less air resistant due to it's thinner diameter. Sometimes you can get a thousand feet of it for not much.
Buddy
It Just Doesn't Matter....