We are specialized in porter services. It is very good profit as part of the complete property maintenance that we provide.
The prices go as low as $14 per hour up to $20 per hour and over. The price depends highly on how many days of the week and hours per day the porter will be needed. Another consideration is the area/state where you are.
From our experience the best and optimum porter service is two to four hours per day, depends on how busy is the shopping center/plaza.
If the PM wants presence during all business hours, that's even better for you

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We ask our porter guys to open there own DBA and we use them as sub contractors. Otherwise the workers comp is too much of a headache.
My own technique to check on my porter guys is to have them take a picture of the plaza with date and time stamped, every 45-60min. from each corner. If the plaza has a second floor, I require a picture from there too. This way they are afraid to cheat and keep the plaza clean

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At the end of the month, we provide our PMs with complete property inspection report with pictures, notes and recommendations. This is extremely good way to propose many other services around the sweeping and porter. It also shows a special attention to each plaza.
If you need sample business proposals or monthly inspection report let me know (as long as you are not in Florida)

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D.