well, after just checking CRC, the highest price Mavic you can get is the EX 729 at 1p under 

but some of slightly cheaper rims you could consider are the XM 719, XM 819 and EX 721, or could go for some of the even cheaper rims, not sure how set you are on gettin the best rim possible for your money or....? 

so...

In my oppinion either 721 or MTX, Though the mtx it around £40 compaired to the 721 which is around £20-£30 or something ?
Correct me if im wrong, i havnt checked prices of rims for a while.

i dont want to sound dumb or anything.  but would a trials rim fit a dmr exalt frame?

Well if you were talking budget, I just picked up a 32hole NC17 Lupus from CRC for 14.99 been pretty impressed by it. As others have said £40 isn't really budget, so if you're spending that much go Mavic, I'd say a 729.
But in all actuality, a wheels strength comes down to the build, get a solid wheelbuilder and you'll get a solid wheel.