I'm probably not the only one who has suggested this, but I believe that pillars should cost less to upgrade, due to the number of pillars/towers in decks, and the fact that they are so essential.
Upgrading pillars is incredibly unrewarding, and only really makes a difference when all/most of them are upgraded. I'd suggest a reduction to around 1000 electrum per upgrade. As this would create a lot of outrage among players who have already spent good *virtual* money upgrading them, they'd probably need to be issued 500 (or less) electrum for each tower they own.
Any thoughts?