Drop rates for renewal version of this event:
Ryujin King 40/3 Diamond(5) Wilfred(15) Plat Fairy(5) Elaine(7) Dragon Crystal 2*3(65)
Invasion3 40/3 Diamond*2(5) Russel(15) Gold Fairy(10) Crave(7) Dragon Crystal 2*2(95)
Invasion2 35/3 Diamond(5) Leo(15) Gold Fairy(10) Cecily(7) Dragon Crystal 2*3(60)
Invasion1 35/3 Ruby(10) Roy(15) Silver Fairy(15) Soma(7) Dragon Crystal 1*3(100)
Dragonewt 35/3 Ruby(10) Misha(Elaine for EN)(7) Silver Fairy(15) Bernard(7) Dragon Crystal 1*3(80)
Dragon Caller 30/3 Crystal(15) Hector(15) Bronze Fairy(15) Phylis(7) Dragon Crystal 1*3(80)
Invasion from Below 25/2 Crystal(15) Bergen*2(15) Bronze Fairy(15) Mortimer(7) Dragon Crystal 1*2(70)
Enormous Hole 20/2 Bouquet(20) Ars(15) Iron Fairy(15) Harissa(7) Dragon Crystal 1(70)
Numbers in (brackets) are the drop rates in %. (ie. 7 = 7%)
Can ignore the dragon crystal drops since they don't drop in this version.
So to get a silver in this event, assuming the drop rate hasn't changed from 7%, you'd have to run a map 14-15 times on average to get 1 drop.
That's about 42-45 stamina if you're running one of the 3-stam maps.
If each sacred crystal you spend to refill stamina recovers 12-13 stamina (mine is 13 atm @ lvl 117), you'd have to spend, on average, 3-4 crystals to get 1 silver.
So to put it into another perspective, you can spend 3 crystals on Rare Gatcha for chance for random silver (or if lucky, gold+). Or you can spend 3-4 crystals on average to get 1 silver drop from this event (unless you're incredibly lucky).
For me so far, did 26 stam worth of runs, 3-starred all maps - 0 silver drops so far