However, I've also read that they can be value by the income they produce through the gross rent multiplier (GRM) method. 

For example, do you look at the Gross Rent Multiplier (GRM)? 

My buddy just approached me with an offer to invest with him in buying foreclosures, he is a GC and he has done house flipping before, he wants me to invest 30K-50K of my own money and use it to get a 6 month rehab loan from a hard money lender at a little over 11% interest, after the House was finished we would split 50/50, he would also add me to his LLC that we would use for that future buy, the reason I’m thinking about investing Is bc I own my own business which will bring 200K this year in gross profits, the down side of my business is that I work 12 hour a day 5 days a week, and I feel that I’m slowly burning out.

@Ben Leybovich should chime in shortly about 30k pigs...I personally own some inexpensive apartments, and the expenses are most definitely not 5% of gross revenue!

Single tenant buildings (especially larger or national) typically pay their own bills directly to the taxing and utility authorities (NNN leases, typically), or pay gross rent.

(A gross lease and with some utilities covered).Thank you for your time and advice!

The property is vested as an LLC and we would like to keep it that way unless we have no other options.Gross Rents on the 4 plex alone are 2700/mo and the duplex currently brings in an additional 1750/mo in gross rents.Your help is much appreciated!

Your net investment income is your gross investment income (less your fully deductible  investment expense)Your do not have to deduct miscellaneous investment expense from your investment income as tax reform took that deduction away.Bottom line: you can deduct the interest if you itemize but limited to net investment income. 

Here are my quick numbers on the deal:$1,600 gross rent-$570 ish for mortgage-$160 for management (10%)-$128 for maintenance (8%)-$128 for CAPEX (8%)-$128 for Vacancy (8%)=$486 month or $5,832 a year (12.9% CoC)Then you still need to budget for Taxes & Insurance, this would drive down your return further.

They charge 10% gross collected rent , and take 50% first months rent when they sign a lease.