I think I can answer this one.
The short version: We don't offer discounts for TP members.
The long version: If we gave discounts to TP members, obviously that would get around, and people would join TP just to get the discount. That doesn't achieve anything. In theory we could offer discounts to people who have been members of TP for N months or who have made X number of posts, but that would be too difficult and time-consuming to administer.
So instead we try to keep our prices as low as possible for all our customers, rather than offering a patchwork of discounts to each special group.
Plus we PAY to support TP, so you get to enjoy it for free!