Now take the terrain into account -- much of the Ice Wall is probably unapproachable except by air -- and you can trim down the numbers even further.
Strategically locate some helicopter pads (on the wall or floating) and you can have heavy armament on the scene in thirty minutes. Assuming an Apache helicopter can fly 150 mph, in that time they could fly 75 miles, so we would need to place pads every 150 miles, requiring 521 pads. Obviously, if you don't insist on thirty-minute response time, you can do with fewer. Don't forget that the watchmen can see 15 miles away from the wall, so thirty minutes should be more than enough to intercept any boat that tries to approach, snap pictures, and sail to safety.
Of course, this is all assuming that the only means our governments have of detecting trespassers is by looking with their eyes. We're neglecting radar and high-altitude spyplanes, probably with infrared cameras.
We're also neglecting intelligence. Anybody who wants to travel to the ice wall has to leave from someplace, and these someplaces can be watched by agents as well. There aren't too many good places to set on on such a journey from. Then, such expeditions would also have to be planned, and agents could get words about them before they even start. Once they've started, agents could monitor radio transmissions. If they can discover tresspassers a thousand miles away instead of only fifteen, then maybe they don't need so big a force as you say.
Now, I'm going to take this, and say that we need about one-thousand men guarding the wall, again with two shifts each, if we include cameras, infrared, radar, intelligence, and all that good stuff. This is more than I stated previously, but bear in mind that I had been very generous with the amount those men had been paid. Now, seeing as the men practically have no influence (all they do is guard an ice wall; it's not like it's probable they have a lot of friends at all, let alone ones in high places), they don't necessarily need a very grand rate of pay. Therefore I'm dropping my previous estimate to a simple one-hundred thousand dollars per person. That still leaves us with the same amount of two-million dollars to pay for every guard in the wall forces.