I do have an Excel spreadsheet I developed which calculates all this for me fairly easily, including a decreasing sale price (in decrements of $10K).

I had a growing family and rent prices hadn't decreased with the housing market.

If you own a SFH and the tenant leaves, you have no income at that property, but that's not the case for a multi-family if you have one issue with a tenant, which can help decrease stress.

I could purchase this property and start charging for gas and electrick and that would decrease expenses by about $550/mo

(I don't just mean ongoing economies of scale for cap ex).If so, then THAT would be the main reason, rather than mentioning economies of scale for repairs.Also, BECAUSE of those economies of scale, shouldn't that be factored in as less expense per month - therefore resulting in INCREASED cash flow per door for Multis, rather than DECREASED?

Now rents will increase and the portion of mortgage interest will decrease over time so it's not really a static situation but it's a good starting point.

We need input:   Our renter has requested to extend their lease by one year in exchange for a discount--they noticed rents in the area have decreased.

Read Dave Ramsey and increase your income AND decrease your lifestyle.

You get better yields (2.3%) than a money market with negligible decrease in liquidity. 

If ample time has elapsed chances will decrease of that occurring.