Once you get above the outright junk/catalogue BSO level in cycling, the price of bikes increases dramatically faster than the quality does. Expensive bikes are not always any better than cheaper bikes in terms of durability, ride quality, or longevity. A lot of the pricing structure is marketing driven, and essentially boils down to how much the maker of a certain brand of bike believes the customer will pay to have one! The only trend that is fairly consistent is that the higher you get up the price scale you will find bikes tend to be lighter in weight, but the reduction is very gradual and the differences would not even be noticeable to many anyway.
Jumping in at the deep end and buying expensive machinery on the justification that it saves an upgrade later is a dodgy argument, IMHO. You only have to look at the amount of virtually as new, barely-ridden machinery on the secondhand market, to realise that the majority of people who go out and buy a new bike on a whim don't actually stick with cycling long enough to even think about upgrading it in a year's time. This is one of the reasons I generally advocate the purchase of secondhand, not new bikes. There is less money involved so if a rider makes an inappropriate bike choice, or gives up cycling altogether and leaves the bike gathering dust in their shed, they haven't lost so much.