I think my estimation was not correct (cc @boldtrn) :
Based on the log of my latest import:
details:edges:230 649 596(7919MB), nodes:178 082 355(2718MB), name:(458MB), geo:1 859 653 157(7095MB), bounds:-180.0,180.0,-79.81499965820461,83.66130796062889,-987.7899780273438,8800.7802734375
I would estimate that the tower node cardinality is on average "somewhere between 3 and 4". For a regular 4-way intersection we have to save 8 turn cost entries, so the size of the TurnCost table would be:
(3 * int + 1 * byte) * 8 entries * nodes = (3 * 4 + 1) * 8 * 180,000,000 ~= 18GB
In practise this will be less. Also, the full byte wouldn't be necessary to store cost levels but I assume this is the minimal size we have to deal with.