Pricing an option to buy real estate: is there a rule of thumb?

I have been approached by someone who wants to buy a six-month option on some real estate I own. (They are trying to put together an assemblage for a retail development, I think.)

Trouble is, I have no idea how to price an option.

I know this is all subject to negotiation, but I’m wondering if there is any rule of thumb out there (perhaps based on a percentage of purchase price).

Does anyone have any experience/expertise in this area?