If you want a present next Christmas don't be too naughty :- )
I would not rob any of a good sale when they are in dire need of cash, but I think £100 is within reason for a Featherweight, most seem to go for £150, and very fast in my area. With a bit of patience and clever marketing I guess it easily will fetch more. If it's a 222K it can reach 300-400, provided it's in very nice condition, complete with accessories and maybe a few extras.
The 222K has to be the most sought after vintage machine ever. The only machines I know can fetch (quite a bit) more is the early straight stitchers from the 1860s to 1880s, Grover and Baker, maybe a Wheeler and Wilson No 9 in exeptional condition. There are a few rare models collectors are willing to pay way too much for.