Marcus, according to the Graphic Artists Guild's Handbook for Ethical Pricing Guidelines, selling all rights to an illustration work should be 500% of the original price.
I don't know how this translates to a fine artist - but, the fee should be considerable. Your rights to your work are what gives your work its inherent monetary value (aside from the actual value of the one piece). You have the right to create prints of your work, and sell those - you also have the right to recreate the work and sell that.
Honestly, though, the biggest implication for you (and your heirs and assigns) would be if you became a very famous, very sought after artist. Then the client whom you sold all rights to would own a very valuable piece of art, that they could license and sell prints of all day long, and you wouldn't receive anything from it.
Flooding the market with said prints could also potentially devalue your originals.
Hypothetically speaking, of course.
