Bulk shipping to asia in particular is very cheap due to the volume of empty containers going in that direction. Not sure about other places.
Countries that recycle plastic have people pick out the good bits cheaply in hellish facilities, profit off selling the high grade recycling, the rest is "waste waste" to be gotten rid of somehow. Dumping it in the ocean is quite cheap.
The sorted plastic used to be a thing that you could mix with virgin plastic to get a cheaper product.
Since China's waste import bans, I believe it's actually more expensive than virgin plastic, partly because it costs more to produce now and partly because demand is driven by manufacturers who want to have a certain % recycled plastic in their product for marketing reasons.
The higher price incentivises other countries to import and process mixed plastic waste instead. They of course will also have to find a way to get rid of the not-useful portions of mixed plastic waste.