They have to pay 100% import tariff + shipping costs (typically on a pallet basis) so you'd expect to pay a lot more...
I struggle with excess luggage without the lime/tanks, taking my own only makes it worse. What is theexcess luggage rate per kilo? (if you can get it) Do the sums is it cost effective, the charter operators are charging quite a lot per kilo of excess.
I've always stumped up & paid for it, in fact I end up using Daeger DiveSorb because that gets shipped in by the container load & was more readily available.
Actually just back from Red Sea, I saved myself a load of hassle/cost & went OC.... 8kg under my weight allowance
JT