I crunched the numbers when the test server had the summer thing going and it worked out more like 33k....i think i know it was more than 20k.I just worked out that if a player wanted just all the available buildings/scouts/generals available in this event, they could purchase them all for 21955 gems. If BB have a 100% free gem sale, it would work out around £35 give or take a few pence. (if I did my sums right)
Now if you say presents will go for 100gc each...and how much that will cost to make in coin/ how many refills you will need...it would clear out most players coin stash.
In short it would be cheaper to buy them with gems than to use those gems to make coin.
...but you can get all the stuff from coin in game so idk what my point was.