This is a great idea, i think though it should be something that doesnt push F2P players away.
Here is an idea i think would work. A discount on every item in the Cash shop. Let the chart look like:
0-100.00 spent- 3% discount
100.0-200.0 spent 5% discount
etc etc.
This would be as per the life of the account, and would apply to ALL item shop items. Rubies, p tokens, diamonds, special sales. F2P characters would not be affected by this at all. Actually it would push them to find people that DO spend money on the game to buy from cause they would know they get a discount and therefore COULD pay less. Alternately it will benefit those paying for the game and give people another reason to maybe spend some here and there. After all if i got a 10% discount when gifting items that could mean a 10% discount to who is buying
Win for Frogster
Win for paying players
Possible Win for F2P players