Is 50 extra square feet worth 10K

What part of town is this in?

That's $200 per square foot- if it's in Manhattan or tons of places in Brooklyn, it's true.

Although I agree with crescent22 mathematically, it depends. If it's just a longer hallway, no. If it's a huge 7 x 7 foot walk in closet, maybe yes. It all depends on what that extra space really is...

I would also say "it depends." Also, aside from what crescent22 said (what KIND of 50sqr feet it is), there is also the idea that no two apartment are "identical." Even in the same building and in the same line, one is on a higher floor than the other, which contributes to price. If on the same floor, even if they have the same/very similar floorplans, they may have different exposures, may overlook different areas of the block, etc (this can affect price greatly if, say, one is directly over an exhaust vent). Or they may have greatly different views. It's pretty rare that the 50 sqr feet are the ONLY difference. All this being said, it's hard to put an exact price on precisely what the value of this added feature is -- whether it be 50 extra square feet, or any number of other things.

I would say "definitely" yes as most sales price are heavily weighted to square footage, irrespective of the quality of that footage

and if it allows you to make a 3rd bedroom (doesn't sound that way in this example however) it could be worth $500-600k

If it allows you to put in a washer and dryer it's worth $1,000,000.