don't know how accurate this is but:
Year,Monthly Price
2002,$5.00
2008,$5.95
2012,$7.95
2015,$9.49
2018,$10.99
2022,$12.49
2024,$13.99
2026,$14.99
Next time you see jagex spend more money on stupid nonsense, remind yourself that "they had no choice but to raise prices, otherwise the company will go bankrupt!"