From what I've read here on this site, the Flat Earth's circumference is around 75000 miles. Now to keep it a secret, they would have to post the fabled ''Ice Wall Guardians'' around this wall at say, 1 mile intervals. Unless my math is failing that means about 74 999 outposts.
Now let's consider the facilities themselves. They would have to be well built bunkers with watch towers. Include state of the art computer monitored security systems, lighting as well as living quarters with heat, running water food and supplies to last. Add to that snowmobiles, 4x4's and a helipad. Let's not forget these guards need to be geared up with good equipment to withstand cold temperatures and of course, weaponry (with ammo to last) to kill ON SIGHT any who venture to near to get a view of the Ice Wall and illegally report it back to another civilian. We cannot assume they would station one single lonely Guardian, they would have to give him/her some buddies, say 4 of them for a grand total of 5 per outpost. I can hear the FE's crying that to be unreasonable so let's cut it down to 2 people per outpost.
That's 74 999 x 2= 149 998 people to pay and silence as well as ALL their relatives who know about their ''little'' secret. Each outpost would cost in the millions at least. Say we cut costs and use the cheapest materials and equipment we get a figure of 1 000 000 including upkeep, and 1 000 000$ would make these outposts look like little shacks with old Pentium PC's and wood stoves for heating.
74999 x 1 000 000 = 74 BILLION 999 000 000 million. We haven't even started paying our heroic Guardians.
Now I can't imagine anyone wanting to be posted out in the cold middle of nowhere for chicken feed but once again, let's be conservative about it and find the craziest drones for the job at around 30000/year. 30 000x 2 =60 000 x 74 999 = 4 billion 499 940 000 every year.
All those costs is for the Ice Wall Guardians alone. We have yet to touch on the rest of the logistics to maintain said conspiracy. Let's keep that for another day, shall we?

This is starting to get expensive for a conspiracy that is supposed to be financially beneficial.