$150 for a Bernina walking foot is the normal price.
The truth is, many sewing machine dealerships will not take returns (and don't even like to do exchanges). I worked in one for awhile, and it was like the one you are describing. What I hated to see were the adults coming in to buy a machine for their Mom for Christmas; if the Mom didn't like that particular machine, it was a problem returning it. If the box wasn't opened, they would exchange for another machine (*never* any refund for a machine!). If the box was open, though, they didn't want to take it back at all (but would take it as a trade-in on a more expensive machine).
I'm not sure why dealerships are set up like this. I do know that it's a pretty marginal business. I have seen a lot of dealerships around here go out of business, but the one I worked at for that short time is still in business.
Oh, and if it makes you feel any better, I purchased a walking foot for my Bernina (and it was close to $150 even years ago), used it for only a few days, and then accidentally caught a pin on it that I didn't see (this was in the days when I pinned my quilts instead of spray basting). It ruined the spring in the foot. My dh said he could repair it if he could get a replacement spring, but the dealership said it was impossible to order the part. I finally bought another walking foot to replace the broken one, but I waited about a year and purchased it from a different place. (The original was one that did go out of business.)