As mentioned by J Jay, 1/2" of drywall is about R-.45. As for foam board, the R-per-inch varies on with the type of foam material (R-4 for Expanded Polystyrene to R-7.4 to R-8.2 for Urethane, depending on the brand).
One thing to remember, you can not leave foam exposed in a living space. It needs to covered with a fire rated material, usually 1/2" of drywall (I am aware of only one fire-rated foam that would not require a fire barrier. Thermax.).
Another thing to consider with foil faced foam. The foil acts can act as a radiant barrier, increasing performance. To acheive this, the foil must remain refletive (The black painted foil doesn't work) and the foil facing must not be in contact with another material (there needs to be a dead air space between the foil facing & it's covering material). This will result in an R-value "Equivalent" of about 2.5 (I say equivalent because R-valu is a measurement of conductive, not radiant heat loss.)