To me, there are two ways to value a rod: one by what the market, or collectors, will pay for it; & second, what a rod is worth to you on a personal level. Ultimately a rod is just like anything else, in that it's price is determined by what the market will bear based on supply & demand. Another influence on the price of a rod is marketing, trends, & other aspects probably based on angler's emotions.
Supply & demand is an easy explanation. There are just a limited number of rods out there by famous rod makers who are no longer with us. When one of their rods comes to market, it will command higher prices because a lot of folks want the rod & there aren't very many available to buy. This is where you might see those four & five figure prices.
A rod's price to you may be completely different than what the market thinks it should be sometimes. If you have a rod that holds special meaning to you because of a personal connection - your Grandpa's old rod, one given as a special gift, etc - you wouldn't dream of parting with it at any price. If for some reason you found your Grandpa's lost old fly rod for sale somewhere, you might very well be willing to pay more than what someone else would pay for it.
It's a fact that bamboo rods will command higher prices & hold their value more than fiberglass or graphite fly rods. Is this true for all bamboo rods? No. Again, we come back to supply & demand based on who made the rod, etc. Some graphite & fiberglass fly rods will be worth more as the years go by than some bamboo rods.
One mistake a lot of folks make is to assume that if a fly rod is made from bamboo, & if it's old, than it must be worth a lot of money. That simply isn't true. A lot of the rods made by famous past rod makers get those high prices because of the quality of their rods. That's how the got famous in the first place. A cheap, poorly made rod one hundred years ago is still a cheap, poorly made rod today. So the quality of the rod does play a large part in the price, as always.
All this is fascinating & good to know, but to me it's not my main concern in deciding what I'd pay for a rod. If you're buying fly rods based on their investment value, you may be wise with your money but to me it just kinda takes something away from the mystique of the rod. Of course, that could only be because I'm a rod maker. Plus, if you only bought rods based on their collectible value you'd miss out on a lot of good rods. Buying a bamboo rod, for example, from a living rod maker shouldn't be done in the same frame of mind that you'd buy stocks or bonds with. That rod's value may even dip a little while the rod maker is alive based on the maker's output, following, & quality of their work.
Ideally collectors would price older fly rods based on how they work as fishing instruments only. There are some high priced rods that I don't care for & others that I think are very undervalued & over looked. In the end I don't think you can ever go wrong buying a rod that you will enjoy using, no matter who made it or when.