Answers

I would recommend replacing the window.

sealing or adding any aftermarket seals would most likely sacrifice operation 

It can definitely be repaired, it would need extensive spackling, most likely with 2 part Bondo, then sended primed and painted.

this is something that should not take more than 6-8 hours in total, obviously it does require some skill.

It depends on the brand of windows.

Fiberglass, Aluminum or vinyl clad wood windows usually start around 600-700$ per each

vinyl windows start at 350-500$ each