This is a comparison of inequality. Comparisons of inequality just as<em> mas que</em> conveys the idea of<em> more than</em> or <em>menos que</em> conveys the idea of <em>less than</em>. These are used to point out that two things are not equal.
So the Less formula can be written as follows:
<em>menos + adjective/adverb/noun + que</em>
In this sentence we use a noun between menos and que. So:
<em>Gasté</em><em> menos que</em><em> mi madre en ropa ayer cuando fuimos de compras</em>