Isolators in general seem to be expensive (custom
processing, smallish market) but you might find a
better price-point by (say) using a cheapest 5V
logic isolator (or 3.3, 2.5V) and a dirt cheap LVDS
Tx / Rx chip (depending on application). Insisting
on the combo cuts your supply base with predictable
pricing consequences.
And of course there's the question of how much
isolation voltage you truly need (if any).
If a minimum transition density can be guaranteed
on the serial lanes then simpler solutions (like pulse
xfmr, DC-blocked) might apply.