Here is my response.
$10 for a pencil drawing is actually very cheap. If you think about it from a paid-by-the-hour view point, a pencil drawing (shaded and cleaned up) usually takes around 1-2 hours to complete. Minimum wage in the states is $7.25. If it takes two hours to complete, the artist is actually underselling themselves by $4.50. Then you have to work out the cost of materials if it's a traditional drawing or the cost of electric used by the computer and tablet if it's a digital commission, so add another $2 to that, easily. So the artist is underselling themselves by nearly an hours wage. If you go by minimum wage + materials, the artist really should be charging closer to $17 for a simple drawing, no colour.
If you're like me who lives in England yet has their price set in US currency, the underselling is even worse. The minimum wage in the UK is £6.08, which is about $9.58, so two hours of work at the UK minimum wage works out to be $19.16. Add in cost of materials (£2 or $3.15) and you'll find that UK artists who charge $10 for a pencil drawing are actually underselling themselves by over $12.
But let's say that I agreed to take on a $5 commission - just to prove my above point. $5 is worth about £3.20. That's about 30 minutes, based on UK minimum wage. However, we need to figure out material costs - let's say £1.60, to make the maths easier. that's about half of the cost, which cuts work time down to 15 minutes. In 15 minutes, I can draw a stick figure, complete with tail, ears, and paws and even throw in a background of the same quality.
Now, let's take a more "professional" artist than I am - someone who pretty much makes a living solely from selling their art and taking commissions. http://cooner.johntoons.com/commissions/ This artist, as you can see, sells pencil sketches for $25 each. This price, for the skill level, is actually really good.
To sum up, $5 for a pencil sketch is WAY underselling the artist, no matter which way you look at it.
Opinions? Thoughts? Please share!