No gold sink is ever going to compete with the insane amount of gp generated by thieving/caskets/ensouled heads on a daily basis.
The "tax" is probably the best attempt as it will scale with inflation.
I support the seed shop, but I think the price should be revisited as the value of gp decreases, leaving the donation store as the preferable means to acquire them (so the server stays alive and all that jazz).
EDIT: Another method to remove some gold from the game could be charging people to use the teleport wizard. Charging for travel is a common gold sink in many MMORPGs.
Maybe make the price increase exponentially based on total level, so it isn't really detrimental to new players, but for higher total level players the cost is noticeable but fair, possibly 50-100k per tp?