String is free
Yeah, I was about to suggest baler twine. Alan has met me so he'll know how smartly I dress on the hill 
Seriously, nip into any high street charity shop and you'll find belts for a quid or two. As there are usually several charity shops in any town, you should be able topick one up very easily.
Or, of course, buy a branded one for £30. But that's thirty quid less to spend down the pub.