Tier9 isn't really about playing to make credits, it's more about enjoying what's effectively pref MM with tanks that are imho, far superior than even OP tier8 tanks along with being more than adequate to deal with tier10 tanks... most of them anyway, not those usual T10 suspects but then again even most T10 tanks won't deal with those but I digress.
So if you're breaking even at tier9, that's a pretty good deal, so long as you're not blowing credits like Christmas and that depends on, as others outline above, the type of tank but also the type of player too. Tanks that can deal lots of damage with standard pen are going to be fine, tanks that can spot for a living, they will be fine but less reliably so since that's team and map reliant... tanks that unreliably deal damage with standard ammo such as HE slingers like a Type4 or T49 derp will generally be fine most the time whereas any of the tragic standard ammo, 2 key warrior tanks like a T-54 will blow credits like nothing else.
That's the tanks economics out of the way. So far as players go, bad players will lose credits along with gold spammers while better players generally won't lose credits unless they also like gold spamming or running tanks with food. So where a player is good, their tanks standard ammo is good and they STILL lose credits... but why? Well, because they have lots of credits in this case and so they will run their tanks with food for a sharper tank offering, can drop optics for vents and not suffer much view range loss for a lot of performance gain, carry more gold ammo, use it if in doubt, those 50/50 moments where they don't want to be screwed over by RNG giving them the middle finger at crucial moments of the battle and because they can and chose to.... so this is tier 9 economics covered I think.