$500 to $5,000 a month is normal from one system in a good location is a good number.
Realistic earnings from a standard location would be $500 a month and higher keeping your ad prices low like $25 to $100 a month. The system can hold 1000 ads but 50 to 100 ads would be better for your clients and you to manage.
Also selling the right side to a sponsor or more than one sponsor will increase your income also.
The location is the key to large income from 1 system. If that location is very popular you can really make some great money from one system.
Here a spreadsheet I made that you can play with to show you how much you can make depending on ads, costs and number of locations you have. Download here