Gaydar - I am not convinced the hassles of the escort chatroom are worth the money they want in this case, but certainly a free / basic cost profile looking for 'generous' partners would be.
Sleepyboy - yes, it's about ?20/month if you want anyone to see a phone number, but its search facilities are crap. You can't even search for straight escorts, for example. On a basic 'who's in London' browse, you're one of 400+ people, listed by how much you've paid them, with just name, age and photo visible to attract profile views.
Rentboy - doesn't have the same readership as those two, and positively goes out of its way to not say how much they want for any ad, but it's probably more than Sleepyboy for London. That gets you anywhere from page four onwards, but you can say more on the visible listing.
I've suggested that, for what's being marketed, the London gay press is a better use of the money: seen only by people in London, able to say clearly what's on offer, and able to be distinctive.