Fish oil, either from oily fishes or from cod liver is by far the cheapest. In Europe, a typical price is around $0.20 per gram of DHA+EPA.
Krill oil is much more expensive than either fish oil or oil of Schizochytrium, so it is a choice that does not make sense.
The oil of Schizochytrium qualifies as a vegan oil. When it has appeared on the market some years ago, it contained only DHA and a negligible quantity of EPA. Some vendors still sell this old kind of Schizochytrium oil. Then it seems that either other strains of Schizochytrium have been discovered, or they have been genetically modified, and an improved oil of Schizochytrium began to be produced, with a proportion 2:1 between DHA and EPA.
Now the only remaining disadvantage of the oil of Schizochytrium is the price. However, in the last 2 years the price of the fish oil has remained constant, while the price of Schizochytrium oil was reduced by more than 2 times, so it can be hoped that in a few years improvements in the cultivation methods might cause the price of the Schizochytrium oil to match the price of the fish oil. In Europe, the price of the Schizochytrium oil is now around $0.80 per gram of DHA+EPA, sometimes a little less than that.
Whoever buys omega-3 nutritional supplements, regardless of what kind, should better take care to always compute the price per gram of DHA+EPA. There is a huge number of shameless supplement vendors whose products have similar prices with the good products, but which contain very low concentrations of the active substances, so that the effective price per gram of DHA+EPA may be even 10 to 15 times higher than at the decent vendors.