It's going to be really hard to give you a price without knowing what type of machine, how big, what features she wants on it, etc. Your best bet to figure out pricing is to figure out what your mom wants, then shop around.

I have 2 Baby locks Elure Plus and Sofia both sewing & embroidery I also use them for my quilting.

I'd check Craigslist and Ebay. I also keep an eye out for estate sales and shop flea markets too.