Yes, round-trip bus and train fare work out to lower than $15 per day by a pretty large margin. If you were to only go into the city and out via subway/train i.e. two rides a day it would work out closer to around $6 per day which is already lower than the $15 cost which also doesn't include cost of gas and parking making it likely much higher of a cost for cars. Of course many people take the train many more times than that per week but thats where OMNY comes in with a per week maximum cost for using transit.

If you use OMNY there is a max you can be charged per week. Essentially all rides are free after your 12th ride per 7 day period. Since the cost of a ride on busses or subway is $2.90 that works out to a max of ~$35 per week for unlimited rides all over the city via train or bus.

I don't live in NYC anymore, but when I did I could never imagine owning a car given the financial burden not being justifiable, but obviously those that do have one are likely in a much higher tax bracket than I am, or are going there for business purposes.