You can build a nice rep Prince Date for a couple hundred or buy a decent vintage Prince Oysterdate for $500 - $600. If you want to build I have a rep black w/silver stick Prince Date dial and case left over...
The price will eventually adjust accordingly according to the market. If people buy these at current prices this may well be the "market rate", if not the prices will decrease until people start buying them then that will be the new "market rate".