By direct do you mean just fuel and the costs of the 50 hour inspections?
Because that's about all 50 bucks gets you. No insurance, no reserve to overhaul engine, no hangar etc etc.
16 liters unleaded 98 = $24
$10 overhaul allowance ($20K by the time you hit 2000 hours)
$16 left for services, or $1600 per 100 hours.
My average 100 hourly is $550. A 50 hourly is basically an intensive daily inspection in a Rotax powered LSA, no need to even change oil.
So assuming $50 leaves a lot of money in the kitty for if parts do fail and need replacing. Or the 800 hour gear box overhaul. And parts aren't neatly as expensive as in certified aircraft.
Adding insurance ($2500) and hangar ($300/month?) take it the total hourly up to $75. Assuming 500 hours.
More if you are unlucky with parts needing replacement, but not as much as you think.
And these are the figures I get from many LSA owners, including schools. Unless they are all deluding themselves, I am going with these numbers!
Adelaide Soaring Club has run a bunch of Jabiru 160/170 (which they run on expensive AVGAS!) for years at $100/hour with volunteer instructors. They certainly wouldn't survive if the actual cost was more than that!