I remember it being quoted as in the £millions, at least.
As for the rest, I play on artificial pitches a lot of the time with my current 11s team and physio for them too; I haven't noticed a significant increase in injuries over that time compared to my last team that played more on grass. If anything, grass was worse because of slippery conditions etc during the winter (we played on decent pitches too, not many quagmires).
For me though, cost would be too prohibitive. That and we wouldn't have moments like the 'Deep End' sign again, which was top notch.