If you are not using the Accounting software, then keep the same input. Right click on the variety name under the Input Tab/Supplies/Seed and Crop name and select Properties. Change the cost here. Then can either read in the next year planting files, or manually record your planting jobs and new cost will go to the Crop enterprises correctly.
If you have the accounting software, check the Supplies inventory report. If that supply does not show, it has a zero qty. You can purchase the same supply name to get the current cost, before or after recording/importing the seed jobs. Hope this helps.