Frame construction method can make a difference in the prices. Just saying a frame is carbon fiber does not make to a good or great frame, there are some poorly made frames out that get sold a bargain basement prices, and bikes get built by those looking to turn a quick buck using those cheap frames. The manufacturer of the bike may get a bigger bulk purchase discount on their component groups, wheels and other finishing bits and pieces, or it might be just a sell off at a loss. Lots of things determine how these guys price their bikes at all levels.

Tiagra is a good, solid performing group of components, and so is 105. The 5800 105 is an 11 speed, it closely mimics the Ultegra 6800, which closely mimics Dura Ace. See the trend here? Tiagra is the beneficiary of Shimano's trickle down technology way of doing business and it is nice shifting, reliable stuff just a few grams heavier and you'll notice some slightly clunkier shifting when compared to the higher level components.

Your need for large size frame is going to make a difference, as you already know and stated, fewer of the extremes at both ends get made due to fewer customers looking for them. To answer you last question in short, probably a higher level of foramen the $2500.00 bike along with some better quality finish bits like the saddle, seat post, stem bars, and especially the wheel set. Allis conjecture on my part with out actually seeing these bikes you are looking at. Oh, welcome to cycling and to the forums, sounds like you are hooked, welcome to the asylum.

Bill