I'm complete rubbish at pricing, but I can tell you that I love your art style and that I would totally be interested in a simple colored piece or digital sketch. Your work is different from a lot of what's currently being offered, and is very dynamic. That will either go over extremely well (water in a desert) or rather poorly (sushi at mcdonalds?!).

My usual advice is to figure out how long it typically takes you, and set an hourly wage for your skill/experience + some for overhead. hours*wage + overhead

Using myself as an example, it takes me about an hour to do a detailed sketch of a halfbody/bust (bust has more facial/hair details). I would still consider myself an intermediate artist, and in my current situation have very little overhead. I use $12 an hour at my level, since that's the wage of entry-level creative positions in my area. 1*12 + 2 = $14 for a detailed sketch in my case. Your mileage may vary.