If making a painting from a photograph and then selling it, how much money, in terms of a percentage of the sale price, would it be appropriate to agree to remit to the model in the photograph and to the photographer?

This type of agreement is contingent upon sale. No sale, no compensation.

How about in the case of a self portrait photograph, where the model and photographer are the same person.

How much for an oil painting versus a watercolor rendition, or does it matter?

I wish I could help. Most of the pictures I draw are from my own photographs or from pictures given to me for a commissioned piece where the person is already paying me to draw them a picture.

I personally think the price depends on the size and detail of the piece. I don't believe it matters what medium you use...it's the amount of work you put in it. I have gotten anywhere from a hundred bucks for an 18x24 commissioned graphite drawing to 45 bucks (which I asked less but the woman paid me more) for a small 9x12 graphite drawing.

Hopefully somebody here will have a better answer for you.

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.