Your price will vary but these rebates seem to stack for some inexpensive double platinum or iridium spark plugs as low as $1 per plug.
Manufacturer rebate: $3 ea for XP Iridium and $2 ea. for double platinum (mail in UPC required)
Autozone $10 GC with purchase of 4 XP Iridium or Double Platinum purchased through 8/20 (online submission - no UPC).
Example for my area:
Double Platinum: APP5325
$5.19 * 4 + $1.45 tax = $22.21 - $8 MIR - $10 AZ GC = $4.21 for 4 plugs
This is a bit better than what I could do at RockAuto ($3.31 per plug price + $5.99 shipping) where only the $8 mfg rebate applies and given that I'll probably spend the $10 GC on a cabin filter in the future..