I just spent ages trying to work this out, so here's a quick post to remind myself should it ever come up again:
The current value of a Forever stamp is 49 cents at time of writing (check current value with this search)
The USPS have an online calculator to work out postage values located here
To send a post card (with a value of $0) it costs $1.20 per card, or 3x Forever stamps
To send a rectangular letter (i.e. birthday card) weighing 1oz or less (with a value of $0) it costs $1.20 per letter, or 3x Forever stamps
You can legally use multiple Forever stamps on a single item of post to ensure that the postage cost is covered.