How much more significant is it in housing them in the US, or wherever else they're likely to disperse them to?
Troops make a hell of a lot of money being based outside their home country. Also, most bases back the USA can easily absorb the amount of personnel it has based overseas, so with that comes the added savings of not needing to operate these extra bases. Canada saved a lot of money when they closed down CFB Lahr in Germany for example.
I'm kind of shocked that you think they're in Germany for the defence of Germany, and not as part of a broader American policy that is tied to their foreign policy. Those troops aren't in Germany to protect Germany, they're in Germany so the US can project power anywhere in the region on short notice. For example, their bases in Germany handled a tremendous amount of the logistics required for the US adventures in Iraq and Afghanistan.
You're right, those bases did handle a tremendous amount of logistics for the Americans. Won't dispute that. That said, these bases will still be there and after seeing what NATO can build up in Afghanistan from scratch over the course of a year, I'm not worried about unmothballing a base in Germany or elsewhere. These bases are luxuries, not necessities.
You seem to want to continuously ignore America's unique standing in the world, that has existed since 1946. You won't see any other country filling that void, because no other country has the capability to fill that void, not because they don't have the desire to fill that void. Once upon a time though, Britain (and to a lesser extent, the other colonial powers) did in fact fill that void. China has started following the US example by developing the ability to project military power far outside of it's borders. It's what all great powers have always done. It's a requirement if you want to build or maintain your hegemony. One has to wonder that if the benefits aren't that great, why has every great power in human history followed more or less the same blueprint? The Assyrians, the Persians, the Macedonians, the Romans, British, etc, etc, etc.
China? Please. What country has a base that houses 50 thousand Chinese soldiers? No where, that's who. The US has floating military bases and the ability to project power all around the globe and they don't need bases in Germany, S. Korea and Japan to do so. NATO has pushed 4000 soldiers into the Baltics with the capacity for 20 times that number if needed and they don't need a permanent base to do so. Took about a year. Not all that expensive.
As for your point that every great power has done it, granted. In fairness to those great powers, they lived in a time where they had direct military rivals who were constantly pushing to take over their spot. The US hasn't been in a war that threatened its hold on it's global power since 1939-1945, yet it maintains these expensive troop deployments and base operations long after that.
Also, if those ancient powers had nuclear weapons, aircraft carriers and aircraft that can drop soldiers just about anywhere on the planet within 24 hours they might have revaluated the cost/benefit ration of having all these global bases as well.
The bases are a relic of the cold war and in large part aren't required the same way they were even 10 years ago. Maybe a argument could be made for Korea, but Japan and Germany, no.
The American taxpayer truly doesn't understand the benefits the US has received from 70 years of empire. Largely because the spoils go to the class of people they tend to approve tax cuts for.
Ah yes, those pesky plebs again. In all seriousness though, if the President works out a better deal with the Germans who are currently paying 1 billion of the 5 billion it costs the US have 50 thousand soldiers in Germany, I don't see the harm. Germany has a budget surplus of 25 billion euros, they can afford it.
What changes is the American ability to influence the decisions of foreign goverments with shows of military force. That's more or less the entire purpose behind the aircraft carrier program, no?
Yes, and with aircraft carriers and stealth planes and the near limitless potential of the drone program, bases become more and more obsolete.
Germany, again, doesn't even have combat ready soldiers there. They don't use it to stage any operations. It's just there. It's a logistics hub. While that's nice, those can be mothballed until needed and be semi operational within a month, fully operational within two. I've seen Canada build one in the middle of wainwright in a week that served as a hub for 3000 soldiers and tear it down in 3 days. And this is Canada, grossly underfunded, using ancient equipment. America, I'm assuming, would do it better.
So if America (Trump) decides it's cheaper to bring the troops home, don't worry, the American military dominance will continue on unabated. Don't lose any sleep over it.