On the whole, even the higher end bikes are not exactly super expensive. Sure, where they have historical significance things can get up there, but even competition winning or otherwise iconic bikes from the various eras can be acquired for relatively low sums.
When I think about the things my friends get up to with their spare time, I am hard pushed to find one pastime that is cheaper than messing around with old bikes.
I don't object to bikes that have had lots of dosh lavished on them.
My favourite bikes are usually ones that have a personal connection with the owner though, regardless of other aspects.
Can't see the attraction of low end bikes that I would have never wanted when they came out though.
Given the cheapness of early Marin frames, for example, I would go for the top end models, (if I was interested in restoring one of them). Yet I see many spending huge amounts of time rebuilding low end metal. Their choice, of course, but not my thing.
Meanwhile, my not insignificant passion for early Kleins has been almost completely eroded thanks to their current status as a fashion accessory.