Premium has been priced at $5, $25 and $50 but they were reduced by 25% for the period of release of new perks. The sale has since been removed but there was only a sale when the premium perk was announced years ago.
When buying premium upgrades it still says it is 25% off currently, it is fully priced at 5$, 25$ and 50$ now, you should probably fix that when you can