While I can’t find a B70 on Amazon I did find a K70 which looks identical (uses the same stock image too). A quick look up gives me these data points from Amazon:
Released: December of 2013 at an all time high of $676
Lowest price point: July of 2014 $84
Average cost: $450
It actually was a fairly low costing maker from Jan 2014- Dec 2014. It hit one final low between maybe June-July of 2015 but after that it never dropped below $300 again. So if he bought it in 2016 he likely paid at least $400.
I collected this price data from CamelCamelCamel.com Incase anyone cares.