If you look at the ENTIRE spec instead of justhte Shimano monnikers you may find something else. As for the wider question of what is worth how much money, I like that approach but I think you're taking a funny look at it (and I mean truly funny, I am not mad or angry or upset or anything, just amused).
You say the price difference between 105 and Ultegra is $300. Why is that? There is much less difference between 105 and Ultegra than there is between a ZG and a Soloist frame. So why do you blindly accept the 105/Ultegra price difference and discard it on the frame part. Honestly I think you have a very hard time finding any difference between most 105 and Ultegra parts, except on maybe the STI (but even there). Just last week a customer of ours had trouble getting his Ultegra front derailleur to shift properly. The solution I gave him? Buy an old 105 derailleur. He did, and the problem was solved.
For the frames, is there enough difference between a $1000 frame and a $2000 frame to warrant the price difference? That is obviously a very personal issue, and I definitely think it is not necessarily tied to the price. There are expensive frames that are total junk, and less expensive frames that are very good. But it does cost a certain amount of money to do specific things to a frame, just like a DuraAce 10-speed crank is more expensive than a Tiagra crank.
But aside from all that, the poster does one triathlon a year and the rest road rides, why on earth should someone like that ride at 75 degrees the whole year? That makes no sense. Aside from the fact that the geometry of the ZG basically ranges from XXXXXXS - S when you really look at the numbers.
Gerard Vroomen
3T.bike OPEN cycle