Wait how does AEC cost more than t70? , t70 u need to tier up twice 20 fuel for t2, 85 fuel for t3, 70 fuel for t70, t1 for brits is 30 fuel then like 15 for research , then 50 for the aec itself, it comes way faster right..? Unless you are referring to mp?
In terms of cost including tech - you are correct. Which is important because the total cost when you include tech dictates the timing that the unit hits.
But I was referring to the raw cost of the unit (i.e. losing the AEC is more brutal than losing a T70).
I think the major point that many players may not be considering right now is how difficult (and all-in) rushing an AEC in a 1v1 is. I would always buy it against a Luchs, but other than that... I'm not so sure. If you're having trouble with an Ost sniper and aren't comfortable going for a countersnipe (or you don't want to camp behind hedges/buildings with a mortar pit) you could get it to run down the sniper... but either way you are investing a LOT of total MP (basically two squads worth or slightly more than a mortar pit's worth) into the AEC - which will ultimately delay your Cromwell tech significantly (by ~3 minutes).
Rushing AEC in a 1v1... you're just not going to have the map control that you need on most maps. It is entirely possible (but not fun or interesting) for an Ost player to field a PaK by/slightly before the AEC hits. More reasonably, the Ost player should grab the PaK after they see the AEC so as not to waste 320 MP.
If Relic doesn't want to change the cost of the AEC (by moving some of the MP into fuel as I suggested earlier) they could make the "research" take an extra 30 or 45 seconds - to delay the initial arrival without messing with cost.
It may be a desirable change as it probably wouldn't greatly affect 1v1 (the AEC would still hit around the same time as Luchs, if not slightly sooner still) but it might help out in team games where an AEC rush can be pretty EleGiggle.