You can't possibly predict whether an image will be a best seller or not. If someone wants to pay for exclusive rights, then I'm going to say it must be worth something. The price, well, you will have to decide.
Some people here, including myself, have had "sell the rights" requests. Prices have been set at $1000 and it was too much. But I personally was not prepared to give up rights for less. Maybe you are.
As far as removing similars, can you write up a custom agreement to dictate the terms or is someone else telling you the terms? If you are writing the terms, I'm going to say that image only is exclusive. You uploaded similars before this request, so if all similars need to come down too, you will have to add that cost into the equation, too.
And yes, I believe images can be removed from most sites within 72 hours.
Just my two cents. Probably worth less.