To see how the gain is calculated, see lines 20-24 on page 2 of form 4797.   But you could say it's roughly this:

Gain = sales price - (cost - depreciation)

So, in this example:  Gain = 500k - (300k - 300k) = 500k

Which makes sense, because if your purchase price is 300k, and you depreciated all 300k of it, then your gain is the whole sale price.

Case 2 is the same.  Every number is the same because "depreciation" in that formula is the depreciation you claimed... or should have claimed.  So it doesn't make a difference whether it's accumulated depreciation, or just the total of what you should have claimed.  You can't report the sale of a rental property that was rented for any length of time and report $0 depreciation.  Well, you can, but it's not correct, and would be disallowed if audited (which, I know, that rarely happens). 

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Ok, that's not the whole story because that's a simplification, and this is where it gets interesting in my opinion.  Like I said, it's not all actually "capital gains", because recapture isn't taxed at the (lower) capital gains tax rate, it's taxed at your (higher) ordinary income rate (but capped at 25%).  So in the above scenarios, part of it is capital gains, but part of it is recapture, and those are at different tax rates. 

I actually setup 3 scenarios in my tax software (and I added $200k of W-2 income), and the results are interesting.  

• Experiment 1:  If they had been claiming correct depreciation all along and have $300k of accumulated depreciation:  $169,909 total tax due. 
• Experiment 2:  If they never claimed depreciation, and we are refusing to properly claim it on the sale either, which isn't correct, but we're just going to risk it:  $76,000 total tax due.
• Experiment 3:  If you do the correct thing and file a 3115 form to claim all that past depreciation that should have been claimed in the past years:  $75,717 total tax due. 

Note that applying a 3115 form results in nearly the same total tax bill as not claiming the depreciation at all, but it's the correct method and avoids a very costly tax bill and huge penalty in the event of an audit (because you can't apply a 3115 form later once it's past the final tax deadline).   Also note that correct depreciation results in a bigger tax bill when you sell the property, but don't forget that they got to claim all that depreciation all those years.