OMRI is an organic certification program, which is fine as far as it goes, but certification does add costs which raises prices. There is a lot of material available that could be certified but isn't, and a lot of mateial that might not even qualify for certification, but will stillnot cause any ill effects to garden soil.
In my experience, soils, compost,s and amendments are usually sold by volume, and should be, because changeable water content will greatly affect weight. I've seen 3 cubic foot bags of soil amendment largely composed of forest product and manure sell for 9 to 12 dollars but I can get a cubic yard of similar material for between 25 to 40.00 including deliver. Even if I compare the lower price of the bagged material and the higher price of the bulk material, the bulk material is half the price. of course the difference can be greater than that,. depending on numerous factors, especially quantity, and quality. However quality itself, can be quite subjective and will depend on usage, aesthetics, and other needs or preferences.
Also, one additional environmental point for bulk compost is it reduces the amount of waste plastic produced (the bags) considerably.
Personally, I buy both bagged and bulk products depending on the material, the quantity I need, and how much I want to store at any one time, but I would prefer to buy bulk more often and do so when the quantity needed makes it cost-effective.