As a point of comparison, here is a fairly rough calculation on our current property, which is paid off. It is a two family with a 4 BR front house that we rent weekly in the summer, and a 2 BR back small house/apartment that we rent annually. We could make more money if we made the back house a weekly or seasonal, but we prefer to have a long term, reliable, reasonable and excellent tenant on the property full-time. We could also get another $12-20K/year renting in the winter, but we don't want the hassle of winter tenants, which are often transients.
By COC (which is a great band, BTW), I take it you mean the amount initially spent to purchase the property. On that basis, we are currently at 90-100% annually, but we purchased in 1999.
Based on our estimated value of our property (which is probably in the range of $1M (+/- 10%) (we get lots of unsolicited calls/texts to sell, and when we answer we say "sure for $2M cash and no inspections").
Gross rents are about $90K, with about $30K in expenses (lawn, utilities, taxes, maintenance, cleaning for weekly tenants, etc). At a 7.4% mortgage rate for$240K down and $900K mortgaged (not sure that is where rates are at, that's what Google says) the mortgage is $5,428/month. Add in expenses, of $30K, and that is $95K/year in mortgage and expenses, which is underwater by about $5K/year.
We could probably make nearly the same money in some sort of index fund or bond, but we like being landlords and our location where we have roots, and our poor kids will probably have to make a choice when we croak.