What determines which act gets the decorated stages?

[bumpbumplove]

And are they paid for by the agency or music show organisers? 

[lighterxx]

Agency pays music show organisers, based on amount you get the decoration, higher you pay more interesting and complex stage you will have

[No1]

Companies pay the stage dressing for their acts, if they can't'spare much, usually they go with the LED panels and confetti, if money's not an issue, then they can go all out. There was an article out there that mentioned how some companies spend up to 10k per stage (including decorations, styling, make-up and staff), so if a group promotes on all music shows, for a month, they're spending a good 250k to promote on music shows.

[starsocks]

what everyone else said, but I think lowkey their relationship with the network comes into play sometimes

[schue]

Companies.

When StayC debuted with So Bad, they had one of the most low budget stage designs I've ever seen.