Then why not 320/115 for single one and discount for double call-ins?
After all this is P4 level tank that will come much later now.
I would probably say because there's no teching cost required for it? Also the fuel cost for those heavy tanks mean that either (A) - the player sacrificed and midgame by saving to call the heavy tank is ASAP or (B) they are not able to call that tank in right away because they spent fuel already on pIVs etc... it's also less than half the MP cost of a tiger so the ost player is still sacrificing a lot of field presence to get that heavy tank out, even if it unlocks only slightly after the 35/85.