I think what most folks here are missing is a very simple idea, cost is equal to your worth in terms of monetary consideration. Or in a form that makes sense:
$=your value of your time,
time= total time vested into a project.
For example I consider myself worth $20.00 per hour. So a project that takes me 2 hours is $40.00. Of course that can be daunting price to most folks so break it down to a per model cost, which I then rescale to about $3.50 a model. That's very much on the cheap fyi
The issue with this is that when you are really skilled, the time put into painting does not reflect how good the product actually is, and you are not really painting the model for an hour straight due to you having to wait for paint, glue, etc to dry. The actual project might take 3 days because of this, but you only put in about an hours worth of time to get an actual level 4/5 result on a single model. Essentially if it takes you more than 1 month to churn out a 2k army at stage4 you should not do commission painting because we are getting paid by a per model basis, and ideally you should be able to churn out ideally 2-3 2k armies by yourself per month (if doing this for some sort of a living most commission painters I know have regular jobs on the side including myself).
So for a 2k LoA army at level 4/ heroes at 5 which you have to do EVERYTHING for should cost between 904 for no conversions and around 1000 for conversions, and that is not factoring in weather or not you have to use your paint and brushes, putty, etc.
sample 2k LOA army:
25 infernal guard
Rescaling it to 3.50 a model is only viable if you are painting at a stage 1 level and no assembly, priming, basing is required.
Take these for example to get a feel for the market:
$450 for 12 ironguts is about the right price for a lvl 5 showcase quality factor in basing to my original which ranges from 1-3.5 dollars each, additionally possible conversion work and the size of the actual model changes the pricing due to a larger area to paint an Irongut is bigger and more detailed than a troll or a bull for example.
Than other issues arise like weirdos who want assbackwards highlighting, or want you to copy something. When this occurs they are really forcing you the artist to work outside of your comfort zone so either you turn down the job, or up your prices personally I would up all prices by
50% for all copy jobs, and weird not the norm painting requests you will have to determine that yourself.
Then if you really want to make serious money doing commissions, you have to get other painters working for you in a team such as the way BTP works but that is a different discussion.