We will calculate tiers against your hashpower. The amount of WBT needed for each tier is dynamic and is calculated based on the current WBT/ETH price.
1 tier - X amount of WBT equal to 0,12 ETH/ GHs
2 tier - X amount of WBT equal 0,23 ETH/ GHs
3 tier - X amount of WBT equal 0,357 ETH/ GHs
4 tier - X amount of WBT equal 0,57 ETH/ GHs
This means if you buy WBT now and the price goes up, you might have surplus WBT on your account. In that case you would be free to either to sell your surplus WBT for a profit, or use it to upgrade to the next tier and enjoy an increased share price and thus profit. In the event of WBT price going down, you might need to top up your WBT balance or go one tier down.