How much does it cost to repair a dropped (borrowed) 70-200mm f/2.8 lens?

I'm so sorry to hear what happened to the lens you borrowed. But dropped lenses happen. It appears the lens has been severely misaligned due to the shock of being dropped. Lenses can usually be realigned by a skilled repair technician if nothing is mechanically broken and the range of adjustment allows for any slightly bent parts.

This is something not even an experienced do-it-yourself lens tinkerer would want to tackle. While I occasionally will partially disassemble simpler lenses for cleaning (when there's something a lot bigger or more harmful than dust inside), I wouldn't dream of opening up a 70-200/2.8. They're some of the most complex mass produced lenses around. All it takes is breaking one of many flex connectors (which is all too easy to do), or touching one focus position sensor (which one doesn't even know exists, much less what it might look like) with your bare fingers to totally brick many complex zoom lenses.

Since the lens does not belong to you, you need to inform the owner of the lens what happened. It's their lens, thus the ball should be in their court to decide how to make their loss whole. Maybe they have insurance or a service contract? Even if they do not, they should be the one to decide how to proceed.

70-200mm f/2.8 lenses are some of the most complex mass produced lenses on the market. They require a lot of labor time to disassemble, adjust, (partially) reassemble, measure, adjust some more, reassemble, etc. Due to the unique design of each brand's versions of such lenses, you're usually better off sending it in to factory service rather than trying to get it done locally. All most local shops will do with a massively misaligned 70-200/2.8 is send it to factory service themselves and then charge you more than factory service charged them (and would charge you).

I believe the price depends on your location. I've tried to fix mine in the official Nikon service in Prague. It cost me around 100 euros and didn't work out :( But still it's cheaper than buy a new one.