Good question, and something I've wondered before. I've yet to see a "steampunk, vintage, rare, collectable" (etc.) style listing for a bayonet cap fluorescent tube, by which I mean where one is listed for sale at a ludicrous price with all them keywords in the title. I've seen three listings for BC fluorescent tubes on Ebay over the last 3 years or so:
The first was a box of modern-ish Sylvania tubes which were actually just bi-pin tubes which had plastic bayonet caps crimped on the end (from the factory). They were very cheap, IIRC they were not much more than £20 for a full box.
The second listing was for about half a box of Philips white from 1974, I bought those for what I consider a very reasonable price
The third was for a job lot of 90x warm white tubes which someone here bought and they got shared out at a meet for £2 a piece.
To conclude, I don't think they're worth much personally, they're just hard to find