The worth depends on what you think its value is worth.
Ive traded many foil rares for straight trades
ofr other standard rares that i needed.
To me foil rares compared to non foil rares have no significant value difference. Using them in your deck makes little diff if its foil or not. So as i said - it does depend on the worth to the seller.
If your a foil enthusiast and you must have the foils then your value of foil cards is higher then that of standard cards.
Foil commons arnt worth much at all. Same deal for Uncommons.
When im buying cards here is the gist of what i offer:
Commons: depending on what set there from 5-10 cents standard 15 cents foil
Uncommons: 15-20 cents standard - 40-50 cents foil
Rares 50 cents to $20 for standard (again depending on set) - 50 cents to $$$$ for foils.
On average your rares are worth say $1 each. If its a foil id pay $1.50 at most. someone desperate for the rare foil version might pay $2 - $3.