Don't delete data. If you don't have the disk space for it at the moment keep it anyway because before long (when the price of storage halves and the capacity of disks doubles) you will have the disk space for it.
More money can be made; more diamonds and gold can be mined. But lost data can never be regained. Second only to life, data is the most valuable substance in the universe. Make copies of data, in multiple physical locations, to ensure its survival.