Vyvyan Basterd
Adventurer
Why not just go with Copper, Silver, Gold, Electrum, Platinum, Diamond, Gold-Pressed Latimuum, then Unobtainium?
But using Electrum Tier might cause people to associate the tiers with particular editions of D&D. We wouldn't want people to think the site was only for discussing pre-3E D&D.
So as not to offend anyone maybe they should just be named "$3 USD Subscription" and "$6 USD Subscription." Keep it generic. You wouldn't want to actually add any kind of gaming flavor to the names in case it's not a member's game of choice.