Brake fluid should generally be changed about every two or three years no matter what it looks like. Brake fluid absorbs moisture from the air and it does not function properly once it gets too much moisture in it. You normally will not notice any problems until some day when your brakes fail so getting it changed is good.
Sorry I can't help you with a reasonable cost since I don't recall what mine cost the last time I had it done. I believe something like $75.00. That was at a dealer, so I suspect it was higher than most. I had it done there since while I know they cost more, I had to have something else done there and I do trust this particular dealer.