Calculating cost basis for multi-unit real estate purchase

I purchased multiple units of differing types in one transaction. Given they are different types, doing a straight (Purchase Price / Square Footage) calculation doesn't accurately reflect comparable sales for each unit type. As an example:

 

Unit 1, 350 sf, comparable sales at the time of purchase $45k

Unit 2, 650 sf, comparable sales at the time of purchase $55k

Purchased together for $100k

 

In what ways can I assign the cost basis for tax purposes?