With the more expensive boards, you are paying for far more than just build quality. For example, a Comp 16 Pro has massive potential for adjustment and fine tuning that you don't get on cheaper boards, its not just marketing spiel, the Matrix truck really does give you loads of room for manouvre. Example, for kiting, you can have a really firm heelside and a loose toeside, then adjust it for downhilling to be more even without taking the board apart. Also, MBS and Maxtrack spend time researching ways of making their components lighter for freestyle, a trispoke hub with standard 8" tyres weighs the same as three Rockstar hubs with T1 tyres, you wouldn't think hubs and tyres could make that much difference but they do.
I just used one board there as an example, but the point I'm making is that in the £300+ price band you are paying for far more than just build quality, there is innovation and constant development behind it . That's not to say that other boards are in any way inferior, I still ride my Jeep Commando 16 thats years old (replaced the deck once cos I was offered one cheap), I've also recently acquired an Airdeck MkII, basically its horses for courses, different boards for different riding styles....