Not quite true, if there is photographer involved all you need is their permission to paint from their photo and most photographers I have dealt with have no problems signing an agreement if I just give credit to them as being the photographer. If in fact you cannot get in touch with the original photographer if you change the photo you are painting form enough that it cannot be easily recognizable as the photographers work you can use that photo in you art. You do not need a copyright lawyer all you need is communication between you and the photographer and a sign contract or a written agreement.If you're selling a piece of work based upon the photographic work of another who might have a copyright?
I think you would do well to contact the copyright holder.
If you want to make a living selling your own versions of someone else's work, you need a copyright lawyer.
If you're just hanging your stuff, based on such work, on your own walls? It's like the VHS/DVD recording laws.
It's all about the money.
But to answer your question, a percentage of the piece is more than acceptable 20% possibly.
The self portrait would depend if you are the one who is wanting to paint it, if so a percentage still would be acceptable, if it is the person who wants the piece painted then there would be no compensation.
Oils vs. watercolor...oils will go more much higher than water color
my 16X20 oils go for $3600 for a single subject and the watercolor go for $1200 and my color pencil of the same size goes for $280
Hope that helps a bit!