I started replacing the elastic in my cloth diapers yesterday. There are two pieces on the legs and one in the back. The top diaper is old. Look how much tighter the legs and back are on the yellow diaper.
It cost me about $2 total for all the elastic for the diapers. If I estimate my time at an average engineering pay, it costs me about $6 a diaper to replace the elastic, which is cheaper than buying new ones. :)
There are plenty of tutorials online so I won't explain how I did it. I think I will be replacing the elastic about every two years of wear. The Velcro seems great on all my diapers. I don't think I will have to replace that until 6 or 8 years of wear.
Ps. I still love my cloth diapers. More on that later.