Where are you based? Set yourself a reasonable target based on them, the purpose of the work (whether it is reusable or a one off print), how long it'll take you and what resources you need.
Asking between £5-50 an hour are all reasonable, it just depends on how you want to sell yourself. They will more than likely negotiate, don't undersell yourself if you've got talent.
Only you know how long it'll take you, get a project plan together and work out how long you're going to take on each individual output. Keep communications active during your work time and find out exactly how they're getting it printed for the best results.