Our brains typically do judge based on relative change even when it doesn't make a lot of sense - I.e. we'll put more effort trying to save ~$1 on something less than $10 than trying to save $2 or $3 on something that's $100, or $10 on something that's over $1000. The former "feels" like a more significant saving - but a $1 is $1 either way. You can post-rationalise it by saying "but I buy $10 things more often than I do $1000 things", but do you really do 100 times as often?