To echo Chris above, and in broad strokes, comb your records and look at capital projects for upping your basis, and they likely would have come in the form of an assessment, like for something major such as a new roof, new siding, etc or something you paid for the interior, then study up how to adjust your basis or see an accountant..

Interesting valuation you have, the $600 part sounds like a tax assessment or an old appraisal, I haven't seen an investor on BP reach a valuation that comes in at $100 above a round number.Tax assessments are for tax purposes, not an investment or market valuation.If that is a true value, there isn't enough of a spread to make this a deal, by the time you have closing costs you'll probably be at market value, you could go over if that valuation is off just a few % points. 

It is entirely possible the tax assessed value has no correlation to the market value of the home.  

If it is just for your own usage just make sure it is permitted to do so then do what you need to do and do not register it with the city until you are ready to sell it so you can avoid a higher taxation and re-assessment of taxes. 

They probably don't know what "market value" means and 2. their accounting or valuation assessment comes from guru stuff.

Rework your numbers and make sure all of the following are included:Principle, interest, taxes (city/county), insurance, maintenance, management, vacancy, HOA and any other expenses and assessments that are associated with the property.

Thanks for the welcome Guys,My chicago property is in lakeview about a mile south from Wrigley field.I understand that chicago has property tax hike to about 2.2% of assessed value on horizon.  

I think you need to assess how much money you are bringing in in residual income from your real estate rents and then break it down.

So do inadequate reserves, pending special assessments, and deferred maintenance on the common areas.

However, we can appeal the tax assessment next year and have a good chance of getting those lowered which would help the numbers greatly.