Honestly speaking I not that concerned about the costs when it comes to tyres (but I wouldn't want to waste money on stylish alu wheels
). My tyres are exchanged when there are 3 mm left or if they are unevenly worn. I simply wouldn't dream of taking any chances just to squeeze a few extra km's out of them and if you don't go below 3 mm, my claim is that you'll have sufficient grip and to me it seems worse to change the balance of the car by raising the front by 7-10 mm. So I stick to the official version with the better tyres in the rear rather than some homecooked theory!
Regards
Lars