While I may not have the best self-control when it come to buying stuff for my dogs, at least I have the smarts to not pay twenty bucks for doggy door bells. ;)
This is the third set I've made with the same bells I bought over four years ago and some freebie ribbon that was part of the packaging to some new wash clothes I just bought.
Five minutes + $0.
You can make them as simple as you want (knots all the way!), do as I did using some knots and sewing at the ends, or make it all fancy-like and sew EVERYTHING.
Or, you can go buy a premade set. Don't worry. I won't judge.