Ask HN: How to account/charge for web app bandwidth/disk usage?

I'm working on a web app in my limited spare time which, if all goes well, will require storing and serving a large number of small files (HTML, JS & images mainly). I'm going to get a VPS from Linode to start with (and possibly join up with a CDN) but I'm not sure how to estimate how many resources each user will require and whether I'm going to lose money off them. For example, I don't want a $10/mo user costing me $50 in bandwidth. I'll set limits obviously, but right now these are just totally abitrary; they could be far too conservative or far too generous!

I realise this isn't very specific but I was curious how other people handle things like this - especially when you have no idea how take up will be, or average usage loads?