I've had many bikes since I started riding seriously in '95. The most expensive was my Merlin Extralight with full Campy Record parts. I paid a tad over 5K for that bike. It seems to me the price of real nice bikes have gone down a bit. Of course, I see the occasional 7-8K Pinarello and high end Trek's go for that much.
BUT, my custom steel Independent Fabrications Crown Jewel frame was only $1800. Great bike BTW! And my favorite single I ride on is my Lemond steel Fillmore fixie. I paid $700 for that bike, new.
So, when we bought our daVinci Joint Venture 700 with Campy/SRAM parts, I paid $6600. I don't happen to think that was too much. I've seen Santana Beyonds go for over 12K. I know the Calfee's are pricey as well.
So, the question is, how much is too much? Or, how much did you spend for your tandem?