Many artists across the world and spanning many different fields undervalue their own work. Which means clients also undervalue the work of the artist. This is what has given birth to the "Starving Artist" stereotype.
I have heard of too many people who charge less than $5 per hour for their work. Many seem to charge under $3 per hour. They don't even realize they are doing it until they break down the number of hours spent, material costs plus factoring in a little bit of extra for overtime - as we nearly all work more hours than we quote.
Here is my general rule for minimum pricing: Charge your country's minimum wage.
For example in Australia, 1 hour of work is $15. The minimum wage here is $15 per hour. This is the entry level wage that doesn't factor in cost of materials or any sort of skill level.
In the US, it is $7.25 per hour. Which is still too low in my book. Artists should be charging at least $10 per hour at the BARE MINIMUM.
Is there any reason, why you, a skilled artist, offering a service that not everyone can perform, should get paid less than what your country values as minimum wage pay?
This doesn't matter if you are offering Tabletop quality commissions or display quality models. You need to AT LEAST be making the minimum wage rate in the country which you reside. If you are particularly skilled, then you can ask for more. Up your price. Don't be afraid to ask for what you are worth.