The parameters I've been given are either raw land, or square footage retail not currently highest and best use, ie, one story block now zoned for 4 - 6 stories.

Since these are both single family investments, it sounds like your best bet would be to sell to a retail buyer.Depending on the quality of the tenants, you may or may not want to have them moved out before listing the property.

This would be no different than just buying any rent ready / retail house on the MLS. 

Is 30% below retail asking price too offensive/too lenient?

I was wondering if there is some basis to use for estimating what an investor contractor would charge compared to a retail contractor.

After reviewing some of the houses that have sold in the neighborhood in this condition (livable as-is but needs $40k cosmetic for full rehab) I realized that I could probably sell this thing to a retail buyer for $20-30k profit if I closed and just quickly listed it (before $5-6k realtor commission).

I've commented on this site before...of the few tasks I clicked on, the prices they quote are about 20% more than I'd pay for the identical work in the markets where I work.This site is more geared towards retail pricing of renovation tasks...investors should be looking to pay less than retail...

We do mostly retail/office but have a few medium density residential projects as well.

@Ben Hughes : Thanks, my husband and I are buying a new construction at retail...

If you're planning on retailing it, be careful about replacing windows.