According to pudding guy (one of the all-time greats ), his threshold is about a half-cent per mile. At that or below, he goes for it. Above that, he has to think about it.

I take a combination of the per-mile cost and my total opportunities to get miles. If it's a dry spell, I try to get as many as I can, still keeping in mind that at about a penny/mile or above it's still not that attractive.

In those cases where I'm going to have to pay something anyhow (like when I put in a new furnace last fall), I went out of the way to get them to take my credit card. No extra cash, a bit of work, and another few thousand miles.