We charge $2 per unit on top of what we would charge for a one color p/f/p.
Here's how we came up with that pricing:
Crown foil costs about 2 cents per linear inch, so for a huge image area, the material cost is going to be under 50 cents per unit.
You have to screen print the adhesive, which is best done p/f/p through something like an 86 or 110 mesh, and the adhesive will add some material costs as well.
The labor is a bit more intensive though, as there is the labor for the p/f/p, the labor for cutting the foil, the labor for pressing it, and the labor for pulling it, and some quick downtime/delay for the foil to cool to room temp. We found that altogether it takes on average about an additional minute per shirt, sometimes longer depending on the size and complexity of the art.
There is also a higher chance of loss due to pressing/pulling errors with the foil.
So material costs and labor come to around $1.50 with our "shop hourly rate" and we mark it up a bit to account for potential loss and slowdowns. Clients have been receptive to that pricing and love the result. Our pricing changes if the art is multiple complex colors and additives or more care are needed to produce the desired result. We also use it with discharge whenever possible instead of multicolored plastisol as it makes the process easier.