For peace of mind I would replace. Tires are one of the most important parts on your car. You don't want a blow out on a highway. The ramifications of saving $100 are not worth your life.....or anyone else's.
I would highly recommend you replace it but if money is tight, eh, watch the spot carefully. If it bubbles or bulges even slightly replace it immediately. If it doesn't then I think you could skate by for a little while. Do the tires have a lot of life left in them or are they looking like they might get replaced soon anyway?