I'm not sure how representative this may be, but I pay $150 to have brass steam locos painted. The painter is a friend of mine and I'm sure the job he does for me would cost more by someone else. Since diesels are much easier, I expect the price may be in the same area once the friend discount is removed. It probably has more to do with how much work the painter in question gets.
Just remember, it's not the painting that takes time, and costs lots, it's the masking for each different color. A fully assembled loco (steam or diesel) takes lots of masking when there is a complex paint scheme.
Pete
