There's a heuristic at work here which isn't completely unreasonable.
I buy $15 items on a daily basis. If I form a habit of ignoring a $5 savings on such purchases, I'll be wasting a significant fraction of my income. I buy $125 items rarely enough that I can give myself permission to splurge and avoid the drive across town.
The percentage does matter -- it's a proxy for the rate at which the savings add up.
It's also a proxy for the importance of the savings relative to other considerations, which are often proportional to the value of what you're buying. If you were about to sign the papers on a $20000 car purchase, would you walk away at the last minute if you found out that an identical car was available from another dealer for $19995? Would you try to explicitly weigh the $5 against intangibles such as your level of trust in the first dealer compared to the second, or would you be right to regard the $5 as a distraction and ignore it?
It seems to me like it shouldn't matter how often you buy the $15 items, technically. Even if you always bought $125 items and never bought $15 items, your heuristic still wouldn't be completely irrational. If you only buy $125 items, you'll only be able to buy 4% more stuff with your income, as compared to 33% more stuff if you always buy $15 items.
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?