Cost sounds fine, they are worth it, you might not think it now but if the time came that you had to call on it there would be far less grief, i've had people tell me there's no need, most of the time there's not but i'm in the throws of a fighting couple at the moment who were starry eyed telling me they didn't need one, i wouldn't advise drafting it up yourselves, one party could claim they didn't know what they were signing or the consequences of it so if it has all been explained to them from the outset there can be no using ignorance as a defence!