I might base it on the value of the product - if it's a cap, maybe $1-2, a box would be more like $3-$4. (This would be equal to a coupon of the same value.)
Most of the time if they're included in a trade of mine, I just roughly calculate it. There really is no hard and fast rule, so it makes them a bit difficult.
Really as long as both parties are happy with the trade, that's what matters.