Um. It's hard since its partially sticks. But I think maybe 200-300G, depending on how much detail you add?
Or you can always say "You suggest the price you think it should be. But please, do not take me for granted."

You can always say "tips are appreciated!" Almost all menewshian's will tip an artist, whether they said they would or not.

If your art improves or if you feel that its worth more, then debate it among the customers and set new prices. If you have the time for it, you can offer freebies randomly within your art shop too--to keep your thread alive. It'll bring in people to chat for a chance to get free art.