Prossh

[Anoobass]

Can you pay more mana than you need for {Prossh, Skyraider of Kher} or any other creatures that give something extra for the mana spent?

Or must you just constantly kill it to up the commander cost?

[Anoobass]

Yea.

[Giggle the Draco Genius]

You can not pay more than the curren cost for it. Some things may make it cost extra such as casting from command zone, or things that make creatures cost more. Also if you make it cost less through another effect X will be less.

[Anoobass]

Lame...

[Gorzo]

You can not pay more than the curren cost for it. Some things may make it cost extra such as casting from command zone, or things that make creatures cost more. Also if you make it cost less through another effect X will be less.

This is correct.

You can tap as much mana as you want, but only the actual mana cost of the spell/ability is being used/paid. The rest will just float in la-la land until used on another spell/ability or go poof when the phase ends.

[ApexPredator]

As Double O Scotch pointed out on the commander thread you can use {Food Chain} for infinite mana.