Every 1050 Cave Key used will give you a guaranteed Rebate of Gold Treasure Tool Essence. However as Kuro stated above, it is best to utilize the rebate and go for a max of 2k cave key rebate.

~Jib

I don't think it's worth it going for more than 1050 or?

At least if u don't buy a lot of keys with cp/ingots.

For spending 1050 Keys u'll get

100 Extra Keys, 24 Purification Runes, 18 Purification Protection Runes and a lvl 4 Magatama

If u go for 1050 Keys and wait for the next Event, u get that stuff twice (if it's always the same reward)

200 Keys, 48 Runes, 36 Protection and 2x lvl 4 Magatama.

If u spend 2100 Keys instead u'll get

140 Key, 36 Runes, 30 Protection and 1x lvl 4/5

So if u don't get more than 1050 keys between events it doesn't seem worth waiting for 2100 keys, I might be wrong tho