Usage of "Not" with "And" [closed]

Negative-polarity markers (not, never, no) do not extend across an and in this sort of situation. However, they do extend across an or.

Therefore, your statement as written is incorrect; it means "if you don't have a gift idea and you want to pay much...". The don't is not carried across the and (which also renders 'pay much' inappropriate as much can only be used with a negative marker in this sense).

You could, however, write "If you don't have a gift idea or want to pay much"; which means "if you don't have a gift idea or you don't want to pay much".

To get your original intended meaning, you have to duplicate the don't:

If you don't have a gift idea and don't want to pay much ...

It's almost ambiguous. Using brackets to enclose verb phrases, the structure is either

(1) "If you [[don't have a gift idea] and [want to pay much]]."   

or

(2) "If you don't [[have a gift idea] and [want to pay much]]." 

(1) has a coordination of two finite verb phrases; (2) has a coordination of two non-finite verb phrases. In (1), "and" is not in the scope of negation, but in (2), "and" is in the scope of negation. In logical notation, after "if" we have for (1) ~p & q, but for (2) ~(p & q).

I said "almost ambiguous", because there is something wrong with using "pay much" outside the scope of negation. At least in this context, "much" is a negative polarity item, which needs to be in the scope of negation:

"You don't want to pay much."  
*"You want to pay much."

So, really, I think the example has only sense (2), but you have to think it through to figure out that sense (1) is not possible, so overall, it's rather awkward. You might try a logical equivalent of (2), inasmuch as ~(p & q) == ~p V ~q, if that is the intent:

(3) "If either you don't have a gift idea or you don't want to pay much."