David, yes, the glass only can be replaced by contacting a local glazing contractor. They can make a replacement thermopane unit to fit in the existing frame. This can be somewhat of an ordeal, as you will want them to install the glass into the frame, so they may need to take the unit to their shop, leaving you with an open hole that will need covering for a time. It's not unlikely, though, that they can come to your home, determine the size needed, make the unit, and install it into the frame at your home.
One item to definitely consider is the age of the door. Most sliding doors made 15 years ago are solid aluminum frame, which conduct heat and cold. Many will "sweat" or even ice up inside during cold weather. With age, the mechanics and weatherstripping of the door will be beginning to have problems, too.
With that in mind, I would find out what the glass replacement would be, and then weigh this against the cost an entire door replacement, which you should be able to do for less than the $2451 you've been quoted, providing we're talking about a fairly standard 6' or 8' door installed on a conventionally built home. I am a general contractor and recently had a door company install 2 very efficient, high quality 8' sliders for a customer, and they did this for under $3000.
If you are replacing aluminum frame sliders, be sure that the new units are "thermal-break" frames. Now, if these quoted replacement units are Anderson or Pella vinyl or metal clad wood frame doors, the $2450 quote is more understandable.