Albert Barnes Commentary Leviticus 25:15-16

"According to the number of years after the jubilee thou shalt buy of thy neighbor, [and] according unto the number of years of the crops he shall sell unto thee. According to the multitude of the years thou shalt increase the price thereof, and according to the fewness of the years thou shalt diminish the price of it; for the number of the crops doth he sell unto thee." — Leviticus 25:15-16 (ASV)

The number of years of the fruits - that is, according to the number of harvests. The average value of a yearly crop might of course be estimated, and the sabbatical years were to be deducted from the series.