It isn't? Port connections are super expensive
Last time I consciously checked, they weren’t.
This wasn’t recently, I have to admit.
The way they are designed suggests that they could be significant if you have a lot of market members (or some very large market members) that are overseas from the market capital. Maybe I just haven’t had a Britain playthrough in a long time.
Anyway, while port connection costs are nice computationally (just add a shipping multiplier to the output), they are fundamentally flawed.
Even if we somehow accept that there are limited convoys and that they are financed by the government (which is very weird in itself), still, when we consider adding new country to the market, we should think of how the goods flow will change and whether we can afford that. Not whether a self-sufficient partner is large enough to tank our market.
In a hypothetical situation where all prices are the same between two markets, joining them together should be a convoy-neutral action. When it’s not, all sorts of weirdness are introduced.
I still think it’s more convenient and beneficial to add countries to the market rather than trade between markets (maybe this will change in 1.9), but my complaints with the system are larger than that (and are not going anywhere in 1.9, unfortunately).
P.S.: and by the way, unless I either misunderstand some fundamental thing or wrong about how the Irish Sea works specifically, this doesn't really refute my originally-worded complaint. Regardless of whether port connections are expensive or not, they still are the same for Dublin and Sydney for a unit of goods produced (not even "for goods shipped", as nothing like that even exists for intramarket shipping).