Investment Basis on Home turned Rental

[Ed M.]

I went through the How to Analyze Deals Guide and it went smoothly except for Investment Basis and Property Price.

How should I set these on a property that was once my home and now turned into a rental.

Purchase Price: $100k (lived there for 3 years, been renting 6 years)
Down Payment: $0, $0 Closing Costs (seller paid)
Refinanced once, Closing Costs: $2000
If Sold Property Today, out the door profit after taxes: $50k
Loan Amount Remaining: $80k

I am trying to analyze my options between:
1. Selling property and investing the $50k
2. Keeping property and paying minimum mortgage payments
3. Keeping property and paying as much towards principal as I can and continue to rent long term

Everything I am reading and my gut tells me to keep it, and pay off the mortgage ASAP.
It is a simple single family home in a very good and desirable area with little maintenance cost and very low vacancy rate.

What would you assign as the Investment Basis? $50k? $2k? $0k? 
What would you assign as the Property Price? Estimated Sale Price? Actual Purchase Price? Refinance Price?

Thank you for your advice!