HELOCs have variable rates, but can be utilized and paid off repeatedly.  

Just make sure you have your contractor and budget on a tight leash because it could spiral out of control if one of those variables isn't managed properly, and rates will be higher 6 months from now.

I mean the goal is to lock the contract up less than asking and make sure all the variables make the deal a great one.

My variable expenses (CapEx, maintenance, vacancy, property management) are consistently hovering at around $750/month.

It’s at a variable rate and just keeps rising.

The square footage, bedrooms, bathrooms, are quite varied making it hard to compare.Have you tried to make a list using common variables to see if that explains the variation is price for purchase and if that could translate to rent prices. not in a complicated way.example: i was shopping in Germantown and square footage, bathrooms, and which floor a condo was on didn't make too much difference in list price or rental market but the bedroom count did.

So if you have access a variable income best tool out there. 

But beware...you also have to consider the market and what other competing properties are offering.Most prospective tenants are EXTREMELY cost conscious; rental rate is most critical variable...so you don't want to price your units higher than the market and increase your vacancy rates.

So there are so many variable that unfortunately I can not give you a precise answer.

Too many variable constantly changing.