I paint for commission from time to time, and usually keep to painting up spare models I dont plan on keeping and ebaying them. The easiest way is to price your jobs in "levels". Given your skill at painting, which is good, but not amazing, try these basic levels:
Level 1: Basic paint job, little highlighting, basing, clean lines, but nothing fancy
5 dollars per normal size mini (ie, tact marine) 10-15 per larger creature (wraithlord, daemon prince, etc) and 20 per large vehicle (land raiders).
Level 2: This is a more involved paint job, with well done highlighting and more attention to details, usually used on squad leaders, hq's, etc. Double the previous prices or a bit less
The best way to get a feel for your pricing is to see how much people are willing to pay for your skills. I get offers at my game shop from passerbys to build/convert/paint, and I generally turn them down because I dont like to look like a jerk. For example a guy yesterday asked me how much to make him a waveserpent like mine (falcon, lots of plasticard, and nice painting) and I told him I dont really do that. Reason being, I sold a similar, but not nearly as nice one, on ebay for 120 a year or two ago. If I were to tell him, oh, about 140 dollars, he'd think I was a jerk. This is why the easiest way to sell paintied mini's, is a forum like ebay, no ones feelings get hurt, and only the people who believe its worth what you're asking actually bid.
Painting aint exactly a money making business until you're very good, i only do it as a way to pay for myself to participate in the hobby.
Btw, level 1 paint jobs should be 1 hour ish, level 2 should be 2-3 hours (these vary on model size of course, but thats time for a basic mini of normal size) and if you offer a special, commander level paint job, these should take 4 hours +.
The mini you pictured is probably around a level 1. Thats not an insult, if you paint for commission, level 1 is the "good table top" standard. If you wanted him to be level 2: thin down your paints, they look thick, be more precise with your highlights, and you'll prolly want 2 shades of hightlights, paint his eyes, and take a bit more time on the purity seal.
Edit: put some examples below of various level work of mine:
Level 1: Clean lines, nothing fancy, a nicely blended weapon as a touch, thats it. Took about an hour. I'd probably charge 5-6 bucks for this sort of paint job.
Level 2: More involved, robes have 3-4 shades, gems are all done, a more involved paint job.
Level 3(or my "level 3" as level 3 is generally more of a showcase model, im no golden daemon winner) : Model has 8-9 shades of blue hightlights, blends are smooth, scenic base (although this one needs more work), well done eyes, a commander model.