Hmm, it could be something that you have to clear your cache with, there are times where prices don't update correctly for some strange reason. You appear to be relatively new so you probably don't know how to clear your cache yet so...
1: Open up Wargaming Game Center
2: Open up your World of Warplanes section
3: Next to the "Play Game" button there will be a little dropdown arrow, click on this arrow
4: You will see launch options, click on "Clear Cache on Launch" or something of the like.
Hopefully this fixes your issue but, if not then there's something a bit more screwed up than I imagined. Also another thing to note, when you go to purchase the plane it should still display the sale price despite the tech tree showing the normal price, so unless the sale is down, don't worry you can still buy it with a discount to boot.
I do fly Chinese aircraft a fair amount so... *ahem*
CHINA NUMBAH WAN
I even made a custom skin for my IL-10M!
Premium plane reviews can be found here. Special project equipment spreadsheet can be found here.