Theoretically and on average, vintage boards are cheaper than the modern mechs targeted at the 'gamer' crowd. However, that goes out the window when you're talking GreedBay and anything that's currently in vogue, which is Blue Alps right now thanks to months of hype and shill bidding. Good like finding a Blue Alps keyboard on eBay for less than $200 these days, which is more than even the top-of-the-line Razer Chroma or Corsair RGB boards, and it'll probably be dirty and beat up (anything remotely clean and the price goes up to $400+).
But more common vintage boards can be very comfortably had in the $40-$70 range.
It really all depends on what you have access to. If you're like Chyros and have tons of ecyclers and universities within easy reach, then old mechs can be easily obtained for little money. If you're like me and live in a rural area where neither of those things are found for hundreds of miles, well, it's GreedBay markup all the way!