Cycling is just one of those areas where there is a massive point of diminishing returns. Where on the price/quality spectrum you wish to fall is entirely up to you. Cycling is utilitarian for some people, and the exact cost of things, and minimizing those costs, while achieving the utility they want out of it, is important. It's more of a hobby for some people, and hobbies don't necessarily demand a cost/benefit analysis.
So I guess it's up to you. If you just want to get the most bang/buck then that's probably somewhere in the $1000-2000 range.
As far as opinions go, they'll be all over the place, and will depend heavily on someone's focus on cost or their ability to pay. Some folks will draw the line at 105-level parts, while for others it's Ultegra, and for the high-rollers anything less than Dura Ace is just unacceptable. Then the Tiagra lovers come out of the woodwork. For the record I'm in the Ultegra camp. I'm willing to pay Ultegra prices to satisfy my material urges in this cycling hobby, but just can't make the leap to Dura Ace because I'm not Mr. Money Bags. The 105 fans will remind me that the benefits of Ultegra over 105 are slim, and they're not wrong. So is going Ultegra "worth it?" It is to me. It may not be to you or anyone else, because that's just a personal decision with many variables and influences behind it.