How exactly do you work out a price for them?

Hmm, I never seen anyone selling just mysql dbs. They usally go along with hosting ;)

-hostriot.com

I was thinking more of extras - ie with blah account you get 2 dbs but the customer wants additional ones

I'd figure out how much diskspace and server resources he's using and charge him accordingly.

(i.e. if he's got 4 300k dbs, no biggie, if he's got 4 10 mb dbs that he's doing multiple left joins on, then he might need some large charges)