You can get advanced coils for under 5k. Here is a link to one reputable seller. Unfortunately you may need to contact him directly because higher level players will buy him out just to raise prices again.
I would be wary of anyone trying to sell you coils for 7k. They are exploiting the fact that many players don’t know that a typical mining run nets 500-1000 gems, and an advanced coil only needs 7.2 gems to make.