When you first click on the delete button next to a price plan, it will check to see what the price plan is currently attached to, if anything. If the price plan is currently not used, you will be able to delete it by clicking on the delete button.

If the price plan is attached to any users or groups however, and it is a "fee based" price plan, you will need to select another price plan to "move" all the users and groups to upon deletion of the price plan. If it is attached to a user or group and it is a "subscription" based price plan, the "move to alternate price plan" will not work for "subscription" based price plans, so for those you will need to manually un-attach any users or groups that are currently using the price plan.