PLA as a raw material is really cheap - you can get it for as little as a few dollars a kilo - and a lot of the cost of a filament roll is just making the spool, packaging, and shipping it. In the end, the price difference between 250g and 1kg of PLA on a roll is rather tiny. Goes the other way too, you can buy 5kg rolls that come down to just $10-15/kg.
The same reason why a can of cola is so expensive compared to a big bottle.