This is based on what I've seen from watching eBay for a few years - but it is just my opinion.
First I look at general demand for the particular model. For what ever reason, some have a bit of a cult following, and sell for more. From what I've seen, this is the major determining factor in price.
Generally, European machines sell for more than US and Japan ones (Bernina, Viking, Pfaff).
For the most part, Singers sell for more than other US made machines, though Antique New Homes seem to sell for a bit more.
Two Spool, Featherweight and now 301's typically sell higher, though FW seems to be decline and prices on the other two are on the rise.
Attachments, bobbin cases/shuttles, controllers and cords for machines other than Singer can really affect price. I don't include Singer because it's easy to get replacement items (vintage or new stock). Some of the others, if you don't get it with the machine, you may never see them unless you are willing to buy a donor machine that has them.
Other than if you have something that is truly collectable - general condition (as long as it's decent) does not seem to have as big of affect on price. Fully serviced machines add to the value for electric machines, but not as much for treadles. Most are worried the motor/wires are fried, so it's not so much that it will make a great stitch, but that it won't short out the first time it's plugged in.