I would have to see the entire space to give you a good answer but my first instinct is the doors would look great painted sherwin iron ore.

$6000!? I would paint white or light gray, but this depends on the rest of the space and what you're going for. These could either add light to a room or make a room darker (which is what they are doing now).

I would also get another bid, just to see if the $6k is an outlier

So the dors are birch with a walnut stain?

Color Zen is right. How can we make a suggestion with no context?

If they are standard sized sliding doors, you can just order replacements at the Big Box stores.