I have a property that is about ~$2k/month negative using 50% rule (which is fairly accurate on this property).  

Anyway what you say is not accurate.

If the deal is REAL (meaning, all comps are accurate, the ARV is accurate, the cost of repair is accurate, and there is 30% profit to be made by fix-and-flipper), then investors should be ones lined up to buy it, and wholesalers can then assign the contract to the highest bidder.

They are the source and any other source, here for example, is not adequate, not accurate and highly likely wrong..

The longer you hold onto it, the more your costs add up, the more costs you have to recover, and the fewer funds you have available to use to recover them with.Think of it like playing Poker (not because it's gambling...REI shouldn't be gambling). 

But I want to hear from the more experienced folks, how are you guys able to accurately analyze rehab costs?

If you have a multi-family property with varying bed/bath counts for each unit, the figure that appears will be less accurate.

Whether you use a management software program or use commonly-used computer programs, just be sure to keep accurate records.5.

If so was the AirDNA data accurate? 

I would suggest that you talk to a professional so 1) your financial information is accurate  and lead to good decisions 2) you don't miss out on any tax saving opportunity