When doing this keep in mind who your playing with, a long time friend should be fine, but when its a random stranger, check what floor your on and how co-operative/defiant he has been, its very well possible that he might leave right after you pay simply to troll.
however if the next floor is a boss floor then its not unreasonable to pay for someone to get extra assistance yourself, another thing to keep in mind is if multiple people are paying for the same person the costs are split between all contributors.
an example is if 1 person was only paying for the elevator, and another person was paying for everything, both players would pay only 5 energy at every elevator with cost, where the player paying only for the elevator will not cover half of any mecha-knight/gate/revive costs.
one final note, keep an eye on how many times people have died before considering using all, trolls can cause very sudden drops in CE (over 1000) in just a minute of dieing and reviving, much faster in boss fights and arena floors
if an odd number occurs such as 3 people paying for the elevator 1 person will have to shell out 4 instead of 3, the game selects players at random if more then one person is eligible to pay that much.