You can generally find prices when a bike was new somewhere on the interwebz. I don’t think there is much correlation between current market value and original sale price. I bought my quite nice early 2000s Allez Elite for $500 last year (no idea if that is a good or bad price) and it originally sold for ~$1600. Was that money well spent? No idea.
OTOH - if I had held onto my ~1980 Mongoose BMX bike I would be many $$ ahead. I have several friends with nifty old cars they have had for a long time and they are many $$ ahead of what they paid.
Personally, I HATE turning hobbies into commodities. It destroys the enjoyment.