What do you think is the accuracy of these free sources?

I can't really speak to the accuracy of their projections, but I will say that I've gleaned more from studying neighboring listings than I ever did from Mashvisor. 

Purchase $123,500 (if not negotiated down) with 31k (25%) down paymentapprox 4k closing costs (not sure of accuracy)Gross Rents $15,900 - 10% Prop Mgt - %5 vacancy - 5% repairs - 5% cap Ex - $900 yrly Ins - $1104 yrly Prop Tax = Net Rents $13,889 annuallyFinancing would be $92.5k 30 yr fixed at (assuming) 5.5% rate.

If you wanted to assess with greater accuracy, you should deduct the carrying cost of the $33k over the time period it takes to build the new unit (ie, the money isn’t generating income, but costing you ~4% interest).

I have walked through the place and have done my analysis, but would like your opinion on the accuracy of my rehab numbers.Similar properties in the area show a market value of 210k-230k.

Window World has come under fire from the Better Business Bureau about the accuracy of their $189 claim.

Watch for the recorded date of each instrument to verify accuracy.

You also need to understand that it is NOT the most accurate way to figure ARV, but what it fails in accuracy, it makes up for in speed.

Collect the most accurate rental and costs information Accuracy depends on your dilligence.

As you break down your estimate into smaller more manageable pieces such square foot cost for a particular room your estimate accuracy increases.