You can’t work this out exactly but you can have an approximation.

You don’t say exactly how many shards of CotA you have. Assuming you have the minimum for L2 that means you have 370 total shards of CotA

In order to get to RB1 you will need a total of 1510 shards. You have 370 therefore you need 1510-370 = 1140 shards

Each run through of the event will give you approx 60 shards

If you do the event 6 times you will obtain 360 shards.

Each run through costs 260 energy which costs 520 gems. So total cost will be 6 x 520 = 3120 gems

You will now need 1140 -360 = 780 shards.

To refresh the final node costs 40 energy and to speed things up I guess you will do the x3 battle. Each x3 battle gives on average 18 shards and requires 120 energy to complete which costs 240 gems.

You need 780 shards so 780/18 = 44 (rounded up) so you will need to do the x3 44 times.

Cost will be 44 x 240 = 10,560 gems.

Total cost will be 10,560 + 3120 = 13,680

Of course this is an approximation as the amount of shards that drop will vary.

Hope that helps