@Sergio Ramirez
As @Greg Dickerson said, it's about NOI. You need to know the operating income and operating expenses (subtract expenses from income) to get NOI.
Next you need to decide what return YOU need on your money if you were to buy it all cash with no loan. If the required return is 6% then you take your NOI and divide by your required return of 6%, also referred to as you CAP rate. This gives you the value of the property to you.
Sellers will often times try to tell you how their NOI is way too low because of a number of factors and that once you take over you be able to drive the profitability (NOI) up which "of course" will support his much higher expectation on sales price.
Be careful with this as it's almost never the case. I would offer on current operations and not assume you can make the improvements he is suggesting especially on a first deal.
All of this being said, it wouldn't surprise me at all if he has no real financials on the property. If that's the case well now you have a totally new new issue to work through, how to build a budget of expected income and expenses.
Good luck but please be careful and try to find an experienced and successful actual human being as a guide or mentor to walk you through this.
Youtube is great for how to change you spark plugs but usually isn't enough for buying an apartment with no experience.