I was in Target (US discount store) the other day to buy some car supplies. Passed a large box of Tide HE powder at the end of an aisle, on sale for $13. Given that I use 45 ml (3 tbsp) of detergent per average load in my Frigidaire 2140, it would take me a year to go through that box. Sometimes I use 60 ml for a heavily soiled load. The 2140's capacity of 3.5 cu ft was mid-range when it was introduced, but now is on the low end on capacity in the US market, as machine capacities have grown. Still larger than just about all European-sized washers. Launderess has posted that she uses just 2-3 tbsp (30-45 ml) of powder per load.
So if I only use $13-20 of Tide HE powder in a year, who cares about cost? This is based on five full loads per week.
Costco doesn't carry Tide HE powder in my area, so I can't buy a huge Costco-sized box for say $20, but the large size stocked by Target, and probably Walmart, is sufficient. Have never tried Sears Ultra Plus HE powder but have heard it comes in large containers for a reasonable price.