You might have 100k tied up in that depending on the distances. 

Let's say the cut-off point is $1000 and assuming it'll stay the same for the remaining 4 years of this property.

What costs should scale to the price of the purchase vice remain fixed? 

pointless/costly to refinance over and over like that" (because, conventional non-owner occupied loans can also be low deposit).My premise of "only buy bargains" is largely so that each time you qualify for your next (low deposit) mortgage, your Loan-To-Value Ratio should still remain below 80% (even though you might have borrowed 95% or more for each prior one), because you bought bargains in the first place, and/or have value-added to each property. 

The Seller carried 90% of the purchase price, and we privately funded the remaining 10%.

But if there isn't, I guess it will simply remain a black box.

My wife and I refi'd into a very specialized 1st position line of credit that's tied to a zero balance sweep checking account.

We estimate having about $100k in equity, $72k remaining mortgage ($375/mo) and an estimated value of $170k+, granted, in this insane market.Comps in our area are renting for $1500+ per month (we’re within walking distance of a university and booming downtown area).I feel like we’re in a great position to convert to a rental and leverage our equity but I’m not sure how to approach the conversion of our current home and acquisition of our next.

Annoying to re set up payments etc but luckily the guy who helped me get the original loans has remained employed through it all and has helped me navigate awful customer service departments, which just gets worse each time.

The Line of Credit is tied to a zero balance sweep checking account, so the loan is really designed to assist you in paying down the balance, while retaining access to every dollar.