I have no measure of cost as each sale would be quite individual.
There's not much between the two in terms of desirability, I feel. Both equal in terms of craftsmanship.
I prefer the looks of the Seeley bike but that's only the aesthetic appeal, and my own preference of style / shape / geometry.
The Rickman appears to have more space around the cylinder head which may allow the motor to be worked on in the frame. Also it has a fixed rear wheel spindle with adjustment of chain drive made with eccentric / cam type movement of the swingarm spindle.