The ex VAT price of rough sawn Oak should be around £60 per cubic foot, obviously cheaper if you buy it by the boatload but for most people this is roughly what you'll pay for decent quality Oak. I'll happily pay £80 a cube for genuinely first grade, furniture quality boards. If you're a furniture maker bitter experience has taught you to put quality above price, if you're a hobbyist I'd suggest you do likewise. Air dried or kilned doesn't seem to affect prices much, I've seen both at a small premium to the other.
If you want fence post grade Oak then you can get it for £30-40, but forget about using that for furniture as it'll have pith inclusions, thunder shakes, and all sorts of problems. I understand that the guys doing Oak framed construction are actually paying a little less than this as the delivered price for wet, squared French Oak timbers.
Don't get caught out by thinking you'll be getting a bargain paying £25 a cube for American Red Oak. You're getting exactly what you pay for, ugly wood that neither finishes nor works well but at a knock down price!
You currently pay a 10-20% premium for pippy Oak, a 15-25% premium for quarter sawn, and a 10-15% premium for thicker boards of 75mm plus (they take longer to kiln).
Over and above regular Oak you get really premium stuff like Brown Oak or Tiger Oak. There's never a standard price for these and in any event you'd want to inspect each board individually, but as a rule of thumb I'll normally pay £100-120 per cube ex VAT for Brown Oak.
So I'm puzzled at the quoted Robbins price of £170. All I can think is it's a typo or it's for something processed/machined like for example T&G Oak boards?