Is there a specific formula you would use to calculate what the offer?

Any recommondations for a website, form or formula that I could use? 

Does anyone know the buy formula used in the New Orleans, LA?

Unless changed, CA uses 2+1 formula and Los Angeles (Municipal Codes 91.1207 & 91.1208) use: 70 to 119 sqft can accommodate two people; 120 to 169 sqft can accommodate three people; 170 to 219 sqft, three people, and one added person for each additional 50 sqft.So a 360 sqft unit is OK for 5 people. 

I don't know the exact formula, but it's usually between 15 to 25 percent of profits.

Here’s Clark county’s formulas you should be able to find your county’s formula as well..

And how do I know what formula to use?

This is clearly demonstrated in the formula: CAP = NOI/Total Capital Invested. 

Purchase price is $290k Once rents are raised to $750 the value would be $310k using the Cap/NOI or what ever formula I found. 

@Mason Hickman I'm not sure which formula was used to calculate the Cap Rate.