This may seem odd from a free member but, how about a set amount for lifetime memberships?
This is economically feasible when you realize that alot of players who buy a premium will buy for a year or two most before the polish wears off. Some of us will keep playing long after but many bore with a game after unlimited access. Ye might also entice free players to join with gold perks for a lifetime price. It's daunting to realize that (although it is only $20) it is a yearly option. Keep the year option for 20 and add a lifetime option.
My two cents,
James Mayfield