If its a pretty standard type beach cruiser, I would think that a BMX caliper type brake could be mounted using the fender mounting hole. What you posted a link to on ebay is just what I was thinking about for a brake kit. I was thinking $20 would be about right for such a thing.

It would be helpful if you posted a picture of your bike and a close-up of the fork and the fender hole drilled through the crown. Then one of us can hopefully verify such a brake would work.