Adding units to Increase value

[Mike Moran]

I have a unique opportunity to purchase a 4-unit property With VERY large units (1450 SF each).  the market rent for an 750-850 SF 2 BR is ~ $800/mo whereas these units only fetch $950/mo (law of diminishing returns). My question is - had anyone ever converted a 4 unit into an 8 unit?  Essentially cutting the units in half and keeping the common hallway where it is. 

Costs? Ballpark?  Any insight or info?