Was in my local Sainsbury's last night and spotted the 300g pouch of peanut M&M's were down from £3 to £2.
The costco deal is cheaper but not everyone can get them. This also makes them cheaper than the 2 for £3 Tesco have the 165g at.
You may however still be able to get the halloween themed ones at 99p and B&M stores if you have one locally. If not then this may be the next cheapest?