As Steve said, there are some plain cabinets like this - you can find PDF versions of the original brochures online.
I have the same machine head in a slightly more ornate 4 drawer cabinet from 1896. I have not since seen one with decals in such good condition from that time period, although I'm in Australia.
I agree that I wouldn't pay $300. I saw a pristine model 15 with flawless sphynx decals and the seller wanted $250 and I thought that was way too much (I offered him $150.)
I was able to buy my not so flawless treadle for $102 from someone who just used it for display in their lounge room, so it was well kept. The decals are 90% there which I thought was good for it's age and the price. The cabinet is beautiful.
Maybe you could say you're interested in the machine head but really unsure about the cabinet... say you COULD be interested but hem and haw about it being so plain etc etc...? See if that helps get the price down. Good luck!