As long as proper expectations have been set and it is not an emergency, getting there to do this "preliminary evaluation" within 48 hours is still excellent customer service. 

There's no real 'fair' way to truly separately meter the natural gas used for the different units, unless you get into rabbit hole land of re-inventing a wheel and a series of sensors measuring heat and air flow within separate ducts, on your (customer) side.I'll comment on each option individually and your decision will be numbers driven. 

(interior at hot water heater)$55 + 57 for the meter(S) and 900 for the 3g data logger, which is 20-30 per months for 3g service. 150 setup fee for customer billing management and a kick back from the tenant of 5-10 for convenience fees.

Thanks @Dmitriy Fomichenko. i'll follow the advise highlighted and come back with any questions.

Look at the apartments in the area of your condo, these apartments are going to be your direct competition, you'll need to offer something they aren't offering to win the customer between the two of you.    

Prices may be many times that in certain rent controlled situations.My suggestion is to research the laws and customs in your target city.

My customers had a choice.

Second, we had to use a custom-built wood front door.

They are not your partners they are customers and should not be allowed to leverage you.

Qvinci is an option for custom reporting