Schizochytrium oil with DHA and EPA, which is sold as "algal" omega-3, for a lack of a correct word that could be understood by the general population (Schizochytrium is not an alga), is very good and no fish are killed for it.
Nevertheless, it remains at least 3 times more expensive than a fish oil, e.g. cod liver oil (I mean price per content of omega-3 fatty acids, not per volume; when not diluted to fool the customers, "algal" oil has a double concentration in comparison with fish oil, i.e. 5 mL of "algal" oil are equivalent with 10 mL of fish oil).
Taking daily a decent dose of "algal" oil can be more expensive than the daily protein intake required by a human, if that is taken from cheap sources (e.g. legumes and chicken meat). Allocating a major part of the budget for food to a supplement taken in minute quantities seems excessive.
I am not aware of any serious reason for the high cost of "algal" oil. A decade ago, it was much more expensive, e.g. 8 times or more in comparison with cod liver oil. Then the price has dropped to 3 times, and then it has diminished no more, remaining at 3 times for 5 years or more.
I believe that it should be possible to further reduce the cost of "algal" oil to make it an acceptable substitute for fish oil, but it seems that the producers are content with their niche market of rich vegans and they do not make any effort to reduce the cost in order to enlarge their market.
I have taken occasionally "algal" oil, to test it, but as long as it remains a luxury food I cannot use it to replace the cod liver oil that I am taking regularly, despite desiring to do so.
I think that it is a health tax, as many things are. For what it's worth, it costs me 50 cents a day. I'm not sure what semantics about it not being a "true" algae has to do with anything, though. If it's a protist or an algae, I'm not sure what that information does other than muddy the waters for people forming an opinion on non-animal based omegas.
If you consume "algal" oil of 50 cents per day, that must be some kind of capsule with a small amount of oil, e.g. a few hundred mg of DHA+EPA.
This is much better than nothing, but it is far from a daily intake comparable to that of the populations who live in places with access to cheap sea fish, where such fish are a significant fraction of their food (e.g. Japan).
If your target is to match the diet of such populations, that means e.g. 5 mL per day of non-diluted "algal" oil, i.e. a teaspoon of such oil (or 10 mL of fish oil), which contains around 2 grams of long-chain omega-3 fatty acids.
That would be much more expensive when using "algal" oil, at least judging after the prices seen e.g. on Amazon.
In order to not scare the customers, many sources of "algal" oil have a similar price with fish oil, but only because they contain much less omega-3 fatty acids per capsule. If you read the fine print, then you discover the true price ratio.