The mail in rebate for all Delco plugs is in effect for all of 2017.
41-801 the double Platinums are $5.20 a hole, so the $2.00 rebate brings that down to $3.20 a hole.
Iridiums are discounted $3 per plug
Double Platinums $2.00
Rapidfire Performance Single Platinum ($1 per plug)
Professional Conventional ($.50 per plug)
The 41-602's are $1.44 or $0.944 a plug after the rebate.
I just bought six 41-834 plugs for my Atlas Inline 6 cylinder, and they were $10.00 a piece. Not cheap but they cover GM's 100,000 mile warranties these days.