I only noticed this today on my alt when I went to purchase the last 5 bank bag slots for my bank alt char,
Basically I was just spamming the purchase slot button and pressing ok rather fast, and the I seemed to manage to purchase the 3rd/4th slots for 10g Each and the last 3 slots for 25g Each (I'm not 100% sure what they're "supposed" to cost tbh but it seemed I bugged the price a bit.
Basically if you spam the purchase slot and ok buttons really fast it doesn't seem to update the price you pay, I haven't had a chance to properly test this so don't flame me if it doesn't work

Would be nice to get some confirmation one way or the other though.