I honestly think you should charge a bit more! Your work is pretty good, dont undersell yourself :)
Not sure how helpful this is, but an artist I admire (ScottFalco specifically) mentioned that you should at least be paying yourself minimum wage for your work. Minimum wage can be measured at around $10 an hour- so if it takes you an hour to complete a flat halfbody, for example, that's what you should be charging for it.
That's an incredibly loose basis, however- it's probably worth it to start somewhere casual and comfortable, then adjust the prices as you get more used to doing commission work.
(I'm not sure how accurate this info is, since I myself haven't begun commission work yet, so you probably shouldn't take my full word for it, hehe! These are just my thoughts.)