I have two pricing models for 2.5D carvings. If I am doing something very custom for a person, I charge my hourly rate for the computer design time and $1 per minute for machine time (as kept track in the log file). After that it is a matter of time and materials to get all of the other woodworking done for the project.
The other pricing model is based more on perceived value (very subjective theory that takes time studying in your local market) then time and materials to do the project. For example, if I do a cribbage board with 2.5D carving and other little nice features I won't price it to include all the computer time and machine running time. This is because when it sells, then I have already done all the work to make another one. No extensive work other then to glue up and prep the blank to go on the machine and carve again.
Hope this helps a little.