[mikeash]

Isn't the cost essentially zero if you just install them when you're doing other work?

[nailer]

Depends on the price of the rubber bits or the moulds for the concrete ones.

[Sanddancer]

Concrete molds for curbs are built in-situ to deal with the situation of that particular curb, and the rubber bits are at most $200, so the cost really is fairly negligible.