The high priest must purchase the bull to atone for himself and his house from his personal funds. He can't use public funds or a donated animal. This means he must be a man of above average means or exceptionally frugal. This principal extends beyond his finances as well. The state of a man's house (family, business, responsibilities, etc.) effects his ability and worthiness to serve God. The bull that the high priest brought was to atone for his entire household.
Leviticus 16:6

https://soilfromstone.blogspot.....com/2008/04/a-pries