A lot,
. . . . . . A lot,
. . . . . . . . . . . . . A lot !!
(said with a British accent)
Seriously, your very first problem is whether a new panel matches EXACTLY the style of your old garage door. Often there are changes in texture/seal design over time by manufacturer so your new window panel will stick out like a sore thumb or have gap where panels connect. Also, they price window panels by themselves VERY HIGH as they know they've "GOTCHA" when you're looking for just one panel.
You may be better off in long run selling your old door complete on CL, and then just buying the new door that has a window panel. This will ensure all your panels match up.