This is the one thing we/I missed going our first time and I guess most first timers do as well. For example, we didn't even know we needed an importer until we were squaring up the last day of the safari with our payments. Another hunter told us about it and the outfitter confirmed.
If it's a buffalo hunt that's $15,000 for 5 days it really is:
$15,000 for the hunt and buffalo
$1000-$2000 for the shoulder mount if done there
$2000-$3000 in air freight
$1500-$3000 in importing, warehousing, misc fees, and inland transpo.
So yeah, on the high side of those figures that's an additional $8,000 on top of the $15,000 for the actual hunt, or 50-60%.
Good rule of thumb you pointed out. Depending what you end up taking, how many animals, and type of mounts...figure another 60-70% on top of the hunt cost for your trophies to get to your door.
A lot of people are probably like us their first time. Didn't know they needed an importer, didn't know shipping was that expensive, didn't know there was that much involved getting it back.
If it's worth it to the person, go for it. I think a lot of people just don't realize the after-hunt work and cost to get your trophies home.