@ D3nt3,

When you cast a spell or activate an ability, the order is as follows: You announce your action and it goes on top of the stack, make decisions about the action, if applicable, and then pay the associated costs. So in the scenario you suggested, Throne of Geth's ability (proliferate) would be placed on the stack, and then you would pay the cost (Tap the Throne and sac Necropede), officially activating the Throne's ability while also putting Necropede's triggered ability on top of the stack. If each ability resolves, you may put a -1/-1 counter on a creature before you proliferate.

People often float mana before casting spells or activating abilities, because it seems intuitive to do so, but you literally can't pay for the spell or ability until after making announcements about it.