Use an honor box or just collect at the circulation desk. You will spend all your time making change. Repairs are a costly nuisance. Our coin-op broke, but it was not replaced because it didn't generate enough revenue to make it worth while. Consider the noise of coins entering the coin-op and when change is returned.
Librarians having to make change is a nuisance.
Complaints that the machine make the wrong change. It prevents unauthorized use by outside groups using the LMC after hours. Our administration isn't interested in subsidizing a free-use machine. We charge 10 cents and it has not been a problem. I do not make change - it has not been a problem.