Answers

I would recommend replacing the window.

sealing or adding any aftermarket seals would most likely sacrifice operation 

Its sounds like you have a double hung window that slides up and down check to see if the outer top window can slide up then lock it. If the glass is sperating from the frame you can take it to a glass replacement company and they will fix the frame and glass if not vinyl welded

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

Michael's numbers are correct, keeping in mind "start at" and that is for the window itself and does not also include a high quality professional installation, spray foam insulation, high quality caulking job, custom exterior cladding, etc. Installed - if you are getting a quality window from a professional window installation company that does't cut any corners and provides lifetime warranties on the product and workmanship and warranties on the glass breakage, etc - vinyl can range anywhere from $700-1,500 and fiberglass/wood clad $1,200-$2,500 depending on the features and add ons. Big box stores, subs, builders, etc would be a lower price point but you would not get the quality most people are looking for with a replacement project on the windows, workmanship, or warranty. Hope this helps.

Vinyl window replacements can range from $400-$600 depending on the brand.  Capping the windows can be an additional charge.

If you go with a Lowen, Norwood, or Lincoln wood window you could be spending north of $1,500-$2,000/window.

Anderson & Pella wood windows with aluminum cladding can be in the $700-$1000 per window range. 

They vary in price we offer a free no obligation in home quote if you would like one please contact us to set it up.