Hey, your art is great. Since these are traditional pieces of a consistent quality- I'd suggest what you actually do is time yourself to see about how long it takes you to work on an inked head shot versus a colored one. Generally, you want to price your commissions based on what your hourly rate of work is, times how long you take in getting work done. For instance, if your rate is $14.50 an hour (a general average for illustrators) and it takes you two hours to work on a piece of art- you should be charging $29. If it takes you half an hour- then the minimum you can charge is $7.25- though by all means you can charge more than that. It depends on what you want to make for your effort, not what the customer will claim to want to pay. If they could- they'd have it for free!