There's no one answer for this that'll work for everyone.

But $40 isn't really any bulkier than $20. <img src="/images/graemlins/smile.gif" alt="" /> And $20 doesn't go as far as it used to. I remember when I could fill up for $20. Not anymore....

(I'm not saying $40 is enough either. But I think $40 is the new $20, if you know what I mean.)