Calculating value for owner/user investment property

[Eric K.]

Situation:

Someone can buy a quasi-retail/office building that has 4 condo units. Three of the units are lease and one is vacant. The buyer wants to purchase all 4 and then use the vacant space for his own. So part of the purchase will be owner/user and another part of the purchase will be investor.

How would you calculate a good purchase price? I'm having difficulty deciding if you should base the owner/user portion on a hypothetical lease value or if you can value the 3 leased spaces as investment and then the owner/user space on a price/SF basis like a single user purchase.

Thanks