John Gill Commentary Leviticus 27:18

"But if he sanctify his field after the jubilee, then the priest shall reckon unto him the money according to the years that remain unto the year of jubilee; and an abatement shall be made from thy estimation." — Leviticus 27:18 (ASV)

But if he sanctify his field after the jubilee
Some years after it, more or fewer, or it may be, when half way towards another jubilee, or nearer:

then the priest shall reckon to him the money according to the years
that remain, even to the year of the jubilee ;
thus, for instance, if it only required an homer of barley to sow it, and the whole value of it from jubilee to jubilee was but fifty shekels of silver; then supposing it to be sanctified in the middle of the fifty years, or at twenty five years' end, it was to be reckoned at twenty five shekels, and sold for that money, and so in proportion, reckoning a shekel for a year:

and it shall be abated from your estimation ;
not the year of jubilee, but a shekel for every year was to be deducted from the original value of fifty shekels, according to the number of years that had passed or were to come.