The minimum coupon value is $9.99 (or its equivalent in other currencies with the pricing matrix).
Udemy almost always has a sale happening for $9.99 - $11.99 on all courses. I wouldn't want to give e.g. a $19.99 coupon for my course and the students wonder my "discount" coupon is more expensive than the standard public price.
Therefore, is there any other sensible strategy than to make all of your coupons either $9.99 or free?
I never make free coupons but all coupons I have for promotional announcements I price at $9.99 - $14.99. Off of Udemy I no longer price much above this as once the student realises they could likely have got the course for much cheaper they all for a partial refund. Going back a few years my coupons were never lower than $27, and usually full price or $49.99 with frequent monthly sales at this price.
Now I think unless you don't enrol your courses in promotions and you have an audience etc I would just stick to coupons around the Udemy price perhaps up to $19.99.