The comparison of BattleDress to Tanks was stating that they are too weak - i.e. can't withstand lower TL anti-tank/armor weapons - for the price. This has been narrowed down to they are just too expensive, especially given the perception that Imperial Marines standardly have this type of armour.
Since BattleDress starts at TL 13 and the average Imperium is TL 12 this probably isn't the case, in addition to the other reasons mentioned for this assumption to be inaccurate (no OT source, lack of skill availiability).
More relevant -
nothing states the 2 MCr cost of BattleDress is what the Imperium pays - which would still purchase in bulk and further may put restrictions on their availability - using economic penalties to limit local militias.
Even though the suit is not invincible - it still packs a lot of advantages (especially against groups not walking around equiped with crew handled heavy anti-armor weapons or high tech plasma/fusion guns...

) - so, I believe that MGT established such a high price to limit player availability. Also, the players don't need to be up against this type of tech at every encounter...
(P.S. = there is no
real approx cost for an M1 Abrams - there are just way too many combinations spread out over 30 years and to numerous allies. Also, the turrent, electronics, countermeasures, etc. are often seperately priced items modularly applied - and there's a
lot of them. The low end versions even have just one engine
BTW: I was a principal in developing a prototype for testing bondlines on the paper composite shells for the Abrams (moons ago) - and the outfit I worked at had 3 mid/upper end Abrams sitting around for a while - ~$18 million per! Local C5 Galaxy crews told me they could only airlift one top of the line Abrams at a time - even in transport config - lower end versions they can carry 2 in transport mode.)