Tosfos (Megillah 23b) is bothered by this number, ten. We have a rule that “ain beis din shakul,” that a court cannot have an even number of members. How then can a beis din consist of ten people?
The Rashba answers that the case of assessment is different. Here the Torah specifically designates ten people as the required number of participants in the process. When it comes to monetary judgments or capital cases, there is no pasuk in the Torah that counts out exactly how many judges are needed. (See Turei Even who already challenges this assertion based on other sugyos). Therefore, in those cases we avoid having an even number. However, here the Torah itself counts off exactly ten people, repeating the term "kohen" ten times, to indicate that is the exact number required.
The Rashba then adds a “ta’ama l’milsa” that really can stand as its own answer and is a nice lomdus. The rule of “ain beis din shakul” applies to only to courts. When ten people gather to assess the value of property, they are not acting as a beis din – there is no judgment of right or wrong, chayav or patur being handed down. They are not a court, but are just a group of people coming together to arrive at some consensus of value.