I don't think such clocks are that expensive. A quick search shows several companies offering them ready to go. I'd guess that even relative to the engineering

I don't think such clocks are that expensive. A quick search shows several companies offering them ready to go. I'd guess that even relative to the engineering time involved, the cost of the clocks was irrelevant. GPS NTP servers start at $1000, but I dunno how much that delivers. The actual GPS signal is probably accurate enough, but the rest of the hardware chain on a $1000 box might not be <1ms. Google might have also wanted a backup in case antennas are disturbed or GPS signal somehow interrupted? But even then - just because Google chose 7ms doesn't mean everyone else has to. Drop a GPS NTP master at each site and live with that? I wouldn't imagine it'd get much worse than 10-20ms? I don't really know a whole lot about this though.