The sentiment ‘it's not much to ask’, presented as the sole justification of why people should do something you want. When everybody has a ‘not much to ask’ request and they're all different (or, occasionally, when the same person thinks of a different one every week for a year), they add up until collectively they are a lot to ask –
Don't just point out that the cost is low and leave it at that. Show why the benefit (whether to you, to whoever you're asking for it, or to somebody else) outweighs it!
1) It usually *is* much to ask
2) Surely it's up to the person of whom the favour is being asked, to know how difficult it is? I mean "It's not much to ask, can you buy some milk?" has an answer of "no, that's impossible" if the corner shop has just been the victim of a smash and grab raid, you are late for work and you've lost your keys.