Given the number of TTN users (many of which will be businesses) I would expect donations to exceed the actual TTI cost, building up a buffer. Until TTI throw some figures at this it’s difficult to say (how cash strapped are they) but £5-10/month/donation would seem reasonable plus some [more] good will from TTI.
I am sure TTI do WANT to keep things running smoothly as TTN is a good test platform for them and is a good news story for their TTI sales.
Anyway, just a thought as it would bring a basic SLA to the TTN platform.