Your approach is spot on Will_Powered. Charge them the going rate for the work they want done. It’s their own choice then.
I had an ‘experience’ last year when a local IT company asked if I could help them out of a PHP hole. I had a suspicion there was more work to be had from them so agreed to bug-fix their project pretty much at cost. I got their code working again (tho it was still pretty horrible) but since then I’ve refused to develop it further unless they pay the going rate for a re-write to do the job properly.
One thing I never got to the bottom of – why would anyone write an invoicing system for a single office small business in PHP/MySQL inthe first place???