Well @Gordon F. , back in early 2014, I was right about where you were.
I'd spent the weekends in Vegas at NoteWorthy and PaperSource. I had attended a couple of training courses too. In hindsight, they were more "show-business" than "how to" practical advice....but I didn't spend too much on them and I did learn a lot. All told, maybe I had invested $2,000.
Then I made a classic mistake. I assumed I could parlay competence from one arena to another. I had enjoyed a long and successful career in corporate America. I had proven my skills in multiple markets over 20+ years. I had built up my sideline business of 30+ rental homes in 4 years and they were doing just fine.
So, in search of better yields and less work, I turned my attention to notes.... I mean, how difficult could it be? I can run a DCF with my eyes closed, I can operate my 10bii in my sleep. I have a great lawyer and a great CPA on speed-dial. Whatever comes up, we can figure it out. Right? well, not so much...
I mildly underestimated how difficult it would be to build a note business based on what I knew. The real problems occurred with all the things I didn't know. Just like the analogy of the iceberg.....what gets you in trouble, is what lies beneath.
The question you are asking is a very important one. Fundamentally, pricing a fixed income asset is a function of variables like interest rate environment, loan performance history, loan to value, property characteristics, borrower characteristics, state-specific metrics and a laundry list of things you might want to include as variables in your model. @Tim S. provides a comprehensive list.
Consider also that two investors armed with the same information might come up with 2 very different prices for the same loan.
Maybe I have some process advantages or a ready buyer for the loan other some other advantage that will allow me to hit my return target more quickly than you (so I can pay up). Or maybe you have modest targets due to very low cost of funds (so you can pay up).
In short, what I'm saying is that there is no correct price for a loan. Like @Jay Hinrichs says, you want to buy it at a discount but the extent of the discount you need depends on what kind of business are you trying to build. These are the fundamental questions you need to consider before buying paper (performing or NPL) or any investment I guess.
Apologies for the lengthy response but this is one of the most common questions I receive.