I've never fought a PVP war, but I've fought a lot of C3 wars, studied the documentation a lot, and talked with those who have. So, with that caveat, here's some notes on what kind of garrisons you might want. (More PVP-experienced players, feel free to chime in with thoughts)
A garrison can have up to 632 units in it, and those can only be AA batteries, armoured vehicles, tanks, jeeps, artillery, missile interceptors, defensive missiles, and nuclear defence. Garrisons don't tend to fight for very long before being destroyed, so ammunition levels can be kept comparatively low - I'll cost them out assuming 2 rounds of fighting before destruction. Here's the cost(using current WG prices) for one weapon and two rounds of ammo for each of the defensive weapon options:
AA batteries: 39.21M
Armoured Vehicles: 27.22M
Tanks: 17.06M
Jeeps: 5.94M
Artillery: 12.66M
Missile Interceptors: 36.64M
Defensive Missiles: 45.46M
Nuclear Defence: 820.8M
AV, Tanks, Arty, and Jeeps all fire against only a very small selection of offensive units(their offensive counterparts, plus special forces and RDUs), which means they are of very limited utility. Tanks and artillery have virtually identical stats(the only difference is that artillery will die very slightly faster against attack helicopters), so if you're going to use either, use artillery, because it's cheaper. Jeeps are less effective than tanks and arty, but are extremely cheap - only use those for bulking out garrisons if you're poor, because they aren't very effective. Armoured vehicles, on the other hand, are significantly more powerful, and are actually a bit tempting even if you have unlimited funds. If you're being attacked by heavy jeeps, for example, a jeep that survives combat will kill an average of 1.75, and arty or tanks will kill an average of 3.5, but an armoured vehicle will kill an average of 8. They should never be a major part of your garrisons, because there's too many other ways to get attacked, but a few dozen of them can be tempting, if only to block the lowest-cost form of attack your enemy has available.
Defensive missiles are similar in role to the above units, but vastly more versatile. DMs can kill everything the above units can kill(except special forces), but they can also kill all non-strategic missile units - AA missiles, AT missiles, medium-range missiles, cruise missiles, conventional missiles, and even missile ships. Given that what I've seen of usual land/sea doctrine these days focuses heavily on these units, this is important. DM batteries are a lynchpin of your defences, and you should have at least a couple hundred in your garrisons unless you're sure that your opponent is attacking 100% by air.
AA batteries are the other critically important unit to have in all garrisons, since nothing else can shoot down planes. There's not much to say about these, but you should have them in all garrisons unless you know your opponent is attacking 100% by land. Again, a couple hundred of them at minimum.
Missile interceptors reduce the damage done to you by enemy missile attacks(including from stealth bombers, but not precision bombers). They do not kill attacking units, but they do shoot down incoming ammunition, which helps preserve your other units. They can be useful to slow down attacks, certainly, but I'm unsure whether I'd rather have these or just more DM batteries.
Nuclear defence batteries are your only defence against strategic attacks. Without them, your opponent can freely nuke your cities and annihilate your population - even if he winds up losing the war, that'll still hurt a lot. However, they're very pricey and only needed in small numbers. In particular, having 2 on your capital and cities is important, because you need 2 in order to block a strategic bomber attack - one will not defend you. Towns and corporations cannot be attacked by any strategic weapons, so there's no need for NDBs there, and while forts, airfields, and bases can be, the consequences of losing them are not generally high enough to justify spending extra billions on each garrison unless you're fighting at very high levels.
So, let's put together a few sample garrisons. Assume we have a country to defend with the usual capital, 20 cities, and 20 towns, plus one of each base and airfield, 150 forts, and 50 corps, for a total of 247 targets to defend. Assume all units are at 330 quality, for simplicity.
- Capital/Cities: 250 AA, 250 defensive missile, 100 missile interceptor, 30 armoured vehicle, 2 nuclear defence. Total cost = 90.06B per garrison
- Other Targets: 250 AA, 250 defensive missile, 100 missile interceptor, 32 armoured vehicle. Total cost = 84.82B per garrison
This defensive setup will cost you 21.06T to garrison one country. Upgrading it to Q450 will take about 18.7 million weapon upgrades and 232.3 million ammo upgrades, for a further cost of about 14.3 trillion.
Now, in practice, you're going to want more than that. You want reserves, and ideally you'd like the ability to switch your garrisons around to focus more on different units, depending on what you're being attacked with. And this doesn't even include helicopters/interceptors or offensive units. Obviously, though, your ability to pay for all this will be limited, so unless you're sitting on a really giant bankroll, even what I specified above may seem a bit fanciful. Judge for yourself what the best thing to spend your money on is.