Essentially what you are saying is that you are going to be adding "finished" square footage to the home. So to find your answer, you need to determine FIRST, what the total sqft of the home will be when completed and if that total is not Normal for the neighborhood, then it's probably not a good idea. Assuming that there are homes within that sqft, then you just need to know the avg. Price / Sqft of homes in the area, then determine what the Construction Cost / Sqft will be to finish it out. For example, if the new finish out is going to add 500 sqft to the house and if the Construction Costs / sqft is $100/sqft, and the neighborhood sells for $200 / sqft, then you should theoretically profit $100/sqft on the construction.
Same example using Total Numbers would be:
House is 2000 sqft then adding 500 sqft would equal 2500 sqft.
Prior to the addition you'd be able to sell your home for $400,000.
The addition (at $100/sqft) costs $50,000, but now the 2500 sqft house should sell for $500,000.
$100,000 higher minus the construction costs = $50,000.
These numbers are just thrown out there. Rarely is there that much disparity, in fact many times it isn't feasible to do the finish out just because of the TIME it takes and therefore interest lost.