Value is in the eye of the beholder.
You may want to search for listings on FB Marketplace, Craigslist, eBay and the like. Keep in mind that listed price and selling price can be very different. FWIW, I bought my 306 for $20. It had no case, no bits, no manual. Just a cord and pedal.
When you list it, share the machine information: date of mfg, steel gears, aluminum body, needle and bobbin requirements, manual (even if it is a pdf), etc. Good, clear photos. Front, back, bits (not all jumbled up), etc.