If the ruler you really want is only $15, you're lucky. Many are double or triple that. Then look at the longarm rulers, where you pay a lot more for the extra thickness. When I want a ruler (or pretty much anything else) I shop around online. Amazon, Walmart, Craftsy, Connecting Threads, JoAnn's, etc. - and eventually I'll find a good buy. Yesterday I found Bonnie Hunter's essential triangle tool at 40% off at Connecting Threads, and then just before I purchased it they came out with an additional coupon for 20% off the entire sale (if you spent $60 or more). I put yardage for a few wide backs (also on sale) in my cart and was pretty happy.
BTW, I don't look at the % off anymore, only at the price I'll be paying. I've found that 40% off at one store may be less than 50% off at another, since they start at different price points. Also shipping can enter into the equation, so often I don't purchase until I need enough to get free shipping.