but they dont spell cosy and there aren't any B**osy so my guess they got snapped up and would have been at around 500.

Fact is is someone has a cosy and want a private plate, then it isn't a bad one to have. Considering a boggo plate will set you back 200 notes, having one like cosy will fetch more money.

As said, it is what someone is willing to pay I guess and if you hold out, you'll get a reasonable price.