£7bn was the original forecasted "worst case" cost. £850m is the current forecasted total.
The money came out of the Ministry of Defences' budget, not additional spending.
While the cost is spread over a few years, it represents about 1.5% of the MOD's annual budget (so probably more like 0.5% p/a for three years).
The people being relocated are those who worked with the British Militarily, whose lives are (accordingly) at significant risk.
Not only is there an overwhelming moral case but there's a practical one - who on Earth would work the the British military again if they hang you out to dry and accidentally leak their personal data?
I cannot even begin to try getting worked up over the cost.