They are rare but not super desirable among collectors. I see maybe 3 or 4 come up for sale a year but they don't fetch high prices. Maglites are collectible but Surefire's got them beat hands down. The prices are higher and there are many more Surefire collectors out there. Maglite is a distant second.
I have a big Maglite collection and I'm only missing 3 or 4 of their D and C cell lights. Older, rarer lights tend to go for more. Anything from Mag's early days, 1979 through 1981 is worth the most. 7Ds by far get the highest prices at auction followed by anything else that's old and in the box.
By the late 90s Mag was producing 3 million or more of certain models a year so even the rare ones don't fetch crazy money.