Shimano's ten speed stuff -is- very expensive. That said, it's very nice. Right now I'm running 105 10 speed brifters on my road bike and they really do feel 'that much' nicer than the older 9 speed 105 stuff, to justify the increase in price...
As for internal differences, I think the main difference betwewen 105/Ultegra/Dura-Ace nowadays is the material of the levers and the pivots.... ultegra and D-A have more bearings in them, and D-A I think has 'lighter' levers.