Quenstion on comarisons - high/low

[anusha.krishnamurthy]

Manhattan GMAT SC guide - Edition 3

The book states that " In numerical comparison the words high and low, as well as higher and lower should only be used as adjectives "

1) Right: My bills are lower than they were last year
2) Wrong: I spend lower than i did last year
3) Right: I spend less than i did last year.

My question is although the 3rd one is often the way it is most used why is the 2nd usage worng ? In both the sentences the lower / less serve as adjectives dont they ?

Thanks !

[JonathanSchneider]

If you argue that "less" and "lower" are both adjectives here, then note that they are modifying the omitted word "money" (the meaning of which is understood), and we don't say "lower money." More importantly, however, we are not using "less" as an adjective here, but rather as a noun -- this is WHAT we have spent. (You could also argue that the word "less" is used as an adverb, with a slightly different meaning to the sentence.)