I have an 830, and 801 Sport and an Artista 170. Technically the 830 is my mom's, but I use it when I need to and it has been promised to me. Of all of her stuff it is the only thing I want to inherit.
Today she called me from an estate sale and said there was a 930 there, did I want it? Heck yeah! I haven't seen it yet, but I can't wait to sew on it. The price was great, I never stumble across deals like this and even if I turn around and sell it, the purchase is an investment.
I have to agree that anything over $550-$600 for an 830 is high, unless the owner has purchased a bunch of extras to throw in. The presser feet can be pricey, my mom's has a lot of presser feet to go with hers.
Off to go pick up my new-to-me machine!