Maybe consider part-worns?
I've just bought a pair of part worn Michelin Pilot Sport PS2's in 275/35/19, both with 4.5mm tread on them. I got them in separate auctions from separate people, but i've got them both in hand now and paid only £100 for the pair incl delivery!
These tyres are around £320 each brand new, but mine only have around 45% of their legal tread remaining. So 45% of £640 is £288, meaning i've just bought £288's worth of tyres for £100.
An absolute bargain if you ask me.
I've had probably 4 or 5 punctures over the last few years, so i've got no intention of putting £640 of rubber at risk to have to bin it if i get a puncture, so it will be part-worns from now on for me.