Agreed, there's no such thing as a "normal" conversion ratio - it depends on so many different factors, one of them being the source and type of the traffic - 1:1000 can be good for tube traffic, but poor for SE traffic.

When it comes to the paysites I promote combined with my traffic, I consider conversion ratio excellent when it's below 1:300, good when it's below 1:500, normal when it's below 1:1000, and bad when it's above 1:1000.