Raj,
I think there are two ways you can do this. In fact, I would do both and see how you comp each one.
1. Comparable sales approach (Per unit) - it seems like you are familiar with comping duplex's, triplexes, etc. Find a few comparables based on the unit type. For example, if the 5-unit contains 2 bedrooms/1 baths per unit and each one is around 1,000 sqft, look for duplexes which contain the same type of units. If the duplex sold for $200,000 ($100,000/unit) then you would comp the 5-unit at $500,000.
2. Reverse income approach - You can find market rents on zillow, look for comparable apartments, reduce what's on market by $100-250 to come up with a reasonable expectation. This will allow you calculate gross income. I'd say you are probably safe using a 40% expense ratio. So your 'NOI" will be 60% of what your total rental income is. Then you want to see what kind of debt you can secure. For a 5-unit, I would try your local banks commercial lending department. (Make sure to ask for their requirements such as: NW, Liquidity, DSCR, Debt Yield) Lastly, you will want to run an analysis and see where you come in at for cash flow. Assuming you are using a spread sheet, you can calculate what you want to make for ROI and adjust your offer price.
Obviously you will need to adjust each method by accounting for defensive and offensive capital improvements, but these are two methods to use. Feel free to DM me and I can assist!
Cheers and good luck!