I was told once that the difference between spending £500 and £1000 is phenomenal. The difference between spending £1000 and £1500, is trivial - at this stage you save mere grams in terms of weight and component quality is only marginally better.
I have to say that when I bought my current bike (BMC Streetfire) I test rode several bikes. two or three from the £400-£600 range. Compared to the BMC (which I paid about £1100 for) they were indeed not very good. I could feel that not all the power I was putting into the pedals was going to the wheels, an indicator that part of it was going into making the frame flex instead. Gear changes felt clunkier and there just seemed to be generally more resistance.
I'm not a bike snob - up to that point I'd been riding a 25 year old Peugeot racer, but even I in my limited road cycling experience at the time could tell the difference.
So yes - to me I would always spend at least £1000. However, I would draw the line at too much above that. I reckon if you're racing and you really, really know your bikes, and that every little improvement is in some way beneficial then you might want to spend 000's and 000's, but personally for me it simply wouldn't be warranted.