I've tried to combat this one by imagining the item at a variety of different price points, with the same saving. I don't know how you'd measure how much success you were having, though, because obviously no-one who understood this bias would exhibit it in a formal test setting, only in more informal settings where it's harder to compare. You need some way of mixing it up so you can't just do the sum, but the bias emerges from the noise all the same.
If you can't "just do the sum" then there is no apparent bias - you are just making an arbitrary choice if you can't compare them rationally.
From Tversky and Khaneman's "The Framing of Decisions and the Psychology of Choice" (Science, Vol. 211, No. 4481, 1981):
This one's a killer. Money is supposed to be fungible, but these observations really highlight how difficult it is to really behave as if you believed that. So, aspiring rationalists, how might we combat this in ourselves? Maybe it would help to consciously convert between money and time: if you value your time at 25 $/hr, then the cost of a twenty-minute drive is 25 $/hr * (1/3) hr = $8.33 > $5, so you buy the calculator in front of you in either case. So this heuristic at least takes care of the calculator problem, although I would guess it fails miserably in other contexts, I currently know not which.
Another takeaway lesson is to ignore advertisements boasting that a product is currently such-and-such percent off. We don't care about the percentage! How many minutes are you saving?