I have a gambling banner exchange program and I offer a 1 to 1 ratio. I’m not trying to say mine is better or anything, I’m just saying you have to be able to give people something that other programs dont. Most other exchanges offer a better ratio than 2:1, they are usually 3:2 or similar.
I would say that if you have a lot of time to put into it and promote the hell out of it then it might be worth it, but the people who sign up for it are people just starting out whom dont have much traffic.
Banner exchanges arent as good as they used to be, people click more text links than anything on sites, at least thats my experience with it.
Good luck if you do it