Suppose I want to pay someone to draw one or more mathematical/scientific diagrams for a paper or book using TikZ, Asymptote, PSTricks, Sketch, MetaPost, or another TeX-friendly system. Preferably, I should be able to make small changes myself, and possibly even use the commissioned code as a learning tool.
What options are available? What sort of costs are considered reasonable?
Motivation: For one thing, it might be useful to refer people to this question when they ask "draw-it-for-me" types of questions.
Important note: Answers should describe an existing service or advertisement, or else give evidence that no such service exists. This question is not intended to solicit bids (or, for that matter, as a place for advertisement, unless you happen to already offer such a service).