im currently using one from rsbotvps.com
the one ive got has 2GB RAM, and is $25/month. So its not the cheapest, but its incredibly simple to use as its designed specifically for rsbots, comes with the rsbots client etc http://www.rsbotvps.com/billing/aff.php?aff=041
www.hazenet.co.uk $8.50 for a 1gig RAM, half that for a 500MB. Pretty good that's what I'm using, and it's burstable. You have to install most things yourself though, which is pretty easy.