If you are really interested in getting a nice vintage steel bike, stick with the Japanese models. Their prices (values) are relatively low right now, compared to bikes from other parts of the world. Secondly, they used pretty standard parts and components, very easy to find replacements, no special threading, etc.

If you want to learn about values, just scroll through the many pages of valuation comments on this forum and start reading the threads. Decent cromoly framed bikes with alloy rims, alloy cranksets and downtube shifters, with no defects, are in the $150 value area. Over $200 value will likely have higher grade components, with indexed shifting. $300 plus will be more modern with brifters. Touring bikes are more. There are more collectable brands that have higher value. If you are on a tight budget, unless one falls in your lap cheap, you are better off with one of the good, somewhat generic Japanese brands: Univega, Panasonic, some Schwinns, Miyata, Bridgestone, and more.