Do the repair and ammo costs go up a lot as you advance into better tanks?
Considerably. My maxed out 3601H medium German tank costs an average of around 5600 credits when actually knocked out with around 75-80% or more damage. I use the long barreled 88MM on it, the ammo coasts around 1400 credits when I fire around 14 rounds in a match. However, it is not uncommon to score 16,000 credits and 350-400 experience points in a match, when on a premium account (I just spent 2500 gold on a month of premium).
The heavy tanks can be prohibitively expensive to run if you are not on a premium account. My new Tiger, that I am not at all happy with, costs at least 9,000 credits to repair. To quote Moriarty, from the movie Kelly's Heroes, "It's a piece of junk".
My JPIV German TD costs around 4000 credits to repair, and about the same to shoot as my 3601, as the guns are similar long barreled 88MM.
As the number of hit points goes up, the cost to repair follows. The 3601H has 710 hit points, where the Tiger has 1350. The Tiger, not having a lot more armor, and even less in some places, is damaged just as easily, it just takes more to knock it out. The worst case scenario is to have a tank like the Tiger taken out by artillery in one shot, that usually hits 90% or more damage, and costs 11,000 credits to repair. If it happens early in the match, you can have 10,000+ credits of repairs, and only earn maybe 2,000 credits at best. The Tiger doesn't earn as much, so when you have an 8,000 or 9,000 credit repair, and only earn 6,000 to 10,000 credits in a bad match, it loses credits rapidly, even on premium.
I kept my 3601H, my JPIV, and my Pz38H735f, specifically because it can be terribly expensive to run a heavy tank. You may need another vehicle or two in order to play enough to run your heavy. I'll keep the 3601H and the 38H even if I make it to the Tiger II. I may or may not keep the JPIV when I eventually work my way up to the JagPanther, it depends on how the JagPanther does. The Tiger will be sold immediately if I get to the Tiger II.