Imo admins have a responsibility to disable signups to their server if they feel they are unable to fund/crowdsource or lack the ability to expand capacity (lack of time or know-how).

As long as server admins stick to the above it doesn't matter if it costs €10 or €100000 a month really.

Its impossible to say at which point is the burden too much, because every person has their own threshold and every server can have multiple admins responsible for it, or might have business backing it or whatever really.