I've been getting coupon offers from half a dozen of sites.
Is it possible to use a formula like "If there is a number that is larger than 60, then remove". This means that all coupons which cost over $60 would be removed.
There's no way to use a formula in a Gmail filter's query. However, you might be able to create a filter that will match most of the results you want with the OR operator. Here's an example:
AFAIK this isn't possible. If the mails that you receive are spam, click on report spam to reduce the chances of them passing the spam filter.
Else probably build a custom filter to filter out emails containing "Coupon"