Why do you want stuff to be expensive? Helping economy = making prices less thus items easier to obtain.
More coupons in the economy equates to inflation, since there's more coupons to go around. The rental items and the guild hideout will eventually take out these coupons from circulation, thus giving more value to coupons, hopefully, less expensive items, less outrageous prices. Right now, it's just not coupons that's in excess, that's why even weapons are getting cheaper. Some people come back from quitting, also reintroducing some items back.
Trust me it's been worse. Cards up to 60 coupons, weapons up to 120, just cause some people had tons of coupons and just broadcast buying items at like, double the normal price.
What I've suggested is a limited time NPC for when new donation rewards are up, offering the item ingame via an NPC at double the coupon price, e.g. $25 for Zodiac, 500 coupons from ingame NPC. Donors will still be able to make money off stuff they donate for (after the NPC is taken out, or even during, but perhaps 1.5-1.9x so people would still buy from them), putting a peg on prices of donation-only rewards (during the duration or even after, cough Zodiac auroras for quadruple donation price ohplz) but still letting them make a profit, taking MORE coupons out of the economy via players who have enough coupons ingame and are willing to buy it off the NPC (right now there's a lot of players with a lot of coupons).
This is one way. Used to work when the server started (via Forsaken King's parts and Wings [which are now pretty much obsolete.])
This does not tackle excess Elite Sets / Weapons though.