In 2017 I purchased a used 2003 or 04 Airborne Zeppelin, mostly Ultegra 6500. Other than the wheels, it was in really good shape and the initial cost was $630, including shipping. Soon after acquiring it, I purchased a new set of Vuelta Corsa SLR wheels, tires, brake pads and chain. Over time, I have also replaced the cables/housing, cassette, chain rings, handlebar, stem and saddle. So, somewhere around $1200 and the bike is like new, other than the decals. The bike is light, fast, comfortable and handles very well. I think the only draw back for me is the tire clearance, 25mm tires in most brands.
I have been very pleased with the bike. I had wanted a titanium bike for a while, but did not want to spend the amount of money a new one would have cost. However, if I was in the market for a new bike, I would now seriously consider doing that. IMO, the ride and durability of a well put together ti bike is worth the investment.