Take your average order value and your margin after product cost, postage and fees. That is what a first order is worth. Then add what a typical customer spends in the following year, because a customer who joins your email list rarely buys only once.
A worked example
An average order of £40 at a 50% margin is worth £20 on the first order. If customers spend another £40 over the next year through email, that is another £20 of margin. A new customer is worth about £40, so paying £20 to £30 to win one is reasonable, even though it looks expensive against the first order alone.