That's a hard question! I have never sold any of my cards, however when you think of what cards that are mass produced cost..... I would easily pay $2.00 - $5.00 for a handmade creation if I didn't make cards myself. Maybe try to figure out the cost to make each card and then add $ for your time? Another idea might be to check out ebay or Etsy to see what cards seem to be selling for.
Good luck in your new endeavor!