The leather used to make Kincade saddles (and most other cheap saddles) is dyed using a faster and cheaper method which basically doesn't go all the way through the leather. Sometimes the color is simply painted onto the outside of the leather. That is why the color has been rubbed off by your stirrup leathers.
There really isn't a way to replace the color. You possibly could cover it a little with brown boot polish, but that will most likely come off the first time you ride in it again.
A reasonable asking price...maybe $100-150? They're going new on eBay
for $200-300 (some even including fittings).