I know other’s have mentioned the high amount of needed sign-ups..
However can you imagine if you only made the min sign-ups 10??
10 might seem like a low number to you at first but consider the possibilities. Getting 10 sign-up is something that is achievable by hundreds of affiliates. So I personally think you would be better off still overing the prize with hundreds of affiliates trying to get as many sign-ups as possible as opposed to a small handful trying to get 500.
Also keep in mind that making this reasonable to even the small webmasters gets your site out out on the web 10X more… it’s great for branding and it pays for itself in what it would cost your company in advertising.
Just my IMHO 
Good Luck