A: How much do you think it's worth giving to
B: Well, under normal circumstances I'd say about a fiver, but considering that we've just come into all this money.... fifty quid?
A: ...Let's split the difference and call it a tenner, shall we?
I could say a lot of obvious things about how wicked this attitude is, about how they lack any standards for figuring out how much money IS appropriate to give, and about why it makes no sense for the amount of money that it's right to give to increase as they get richer. Instead, I'm just going to say one thing about it.
Look at how the show presumes that the Communist attitude is the right one. The joke is that character A is miserly. It just assumes that character B's attitude is the right, kind, nice one, and that the joke is on A for being unwilling to go along with A's Communism.
I don't think there's any big conspiracy here. It's just a bunch of light comedy writers making a joke (character A happened to be Irish, which made it quite funny). I'm not interested in spitting acid at the writers of The IT Crowd. I'm much more interested in the fact that this is an accepted belief right now.