After speaking with the owner, he was willing to sell it at a price quite a bit below comparables if cash (or cash plus willing to assume the remaining 25k loan).The sale would be done with a quit claim deed and he'd want the equivalent of one month's rent as earnest money.

Here, you can charge an equivalent of one month's rent only as security deposit and the court has subsequently ruled that the "one month's rent" it is equivalent to is the very first month's rent.

A credit card cash advance is sort of the equivalent.

Of course there could be some side arrangement where they pay someone the equivalent of a rent.

I'm not looking for the credit repair equivalent of a get rich quick scheme.

If roughly equal, then think like a tenant -- consider things like parking, heating/AC sources and equivalent cost to heat/cool, noise (eg are the units side by side or above/below, busy roads etc), W/D hookups, how the unit "feels" (a sunny bright open unit will feel bigger than dark one), distance to attractions/bus lines/work/school/whatever.. 

Just cutting the child a check without issuing a W-2, without potentially reporting on the applicable Form 94x series (and state equivalents), without having a completed I-9 and signed contract on file...if I was an IRS auditor I'd deny the deduction, and claim the checks were gifts, and you'd have a very hard time proving me wrong.Unless the child is going to be working a lot in your business, the juice may not be worth the squeeze here. 

@Joseph HammelIf you're hiring professionals at these rock-bottom prices ($650 for your business is ridiculous - an equivalent of day labor prices for construction) - then good for you.

In other words each month of vacancy is equivalent to lowering rent by $129.

Purchase price was 135k and I put around $120k into it.Also created a laundry room and vending area which I hope will bring in equivalent to an efficiency unit.