I would guess that the CTR is going to be low but if targeted to the right niche, you may be able to get a few signups. Without knowing exactly what products/services you sell on your other sites, it’s hard to tell.
If you have a shopping cart, you could do something that Amazon does at checkout. If you are buying a product in a certain category, they offer similar type products. If you’re selling decks of cards or a poker table, you may want to also offer a casino/poker room offer.
Good luck.