Spoken like a true 'Plank' (
Feel free to look up the reference boys and girls.) You need to stop thinking like a soldier and start thinking like a corporate executive.
Yes, the military does most of the fighting today, but in the dark future the streets of most major cities are more dangerous than some warzones. There were 4 corporate wars, (wars fought between corporations with private armies,) in less than 30 years in the Cyberpunk world. (Ok, the 3rd war was virtual, but it still counts, kind of.) The militaries of governments does not fight the same was as the military of a corporation.
To get into the correct mindset, I would appreciate it if everyone read the following
FAN WRITTEN WARHAMMER 40K DOCUMENT. It is a true work of art and totally 'on the money'.
On to the main course:
Currently, the production cost of an M1 Abrams is about $8.6m, (so almost double your estimate.) And the far superior, (imo,) Challenger 2 comes in at £4.2m, (or about $6.55m.) Each shot they fire from their main gun is going to cost upwards of £50,000, (or $78,000.) The equivelant tank of the US Army in 2020 is the M-11 MBT which comes in at a staggering 16.4m EB, (or $32.8m.) You can pick up a private Jet for under $1m.
They are not even close to the same price range.
But the biggest cost is not the tank itself or even the ammo, (100 rounds costs more than the tank after all...) It's the collateral damage.
Rolling a tank out is going to cost a fortune in torn up road, but lets say that our trusty tin-can-o-death has specialised treads. What happens when it misses with a round or two, or clips a building on a bad turn? Innocent passers by? You're now talking hundreds of millions in damages, if not billions. A good chunk of that cost will be company property. I don't think that would count as beiong tax deducatble either...
This doesn't even begin to cover the fact that a tank is not exactly a 'subtle' peice of kit. It's not like Mr Arasaka is going to walk into the local police station and apply for a permit for his MBT. Can you imagine the bribes that would be needed to get the authorities to sign off on allowing a private corporation to field tanks in civilian commercial, industrial and residential areas? I believe the best estimate I can come up with is "Shit-Tons o' Money!"
Did I forget to mention that one of the worst environments for tanks is dense urban terrain, due to the limited maneuverability and field of view?
In short, there are much cheaper, simpler and efficient means of achieving similar levels of firepower and defence possesed by a tank that are open to the corporations.
As for that walker, (not plural, there was one,) many of us, (myself included,) prefer to deny its existance. Mike Pondsmith gave a perfect explanation as to why walkers like that just don't work in real life. (Have a look at some of the threads about it, there are several on the forum already.)
Gundams... No. No. Really. No means NO!!!
Iron Man suit. There are forms of it in 2020, they are called ACPA, (Assisted Combat Personel Armour,) or "Power Armour". The issue there is that they are just basically bullet sponges, and the point of the exercise is to avoid that.
Fully Cybered Ninja's on drugs. Now you're thinking like a 'punk.