If you buy the map packs separately they cost $15 each. There will be 4 map packs in total, meaning that if you buy all four it costs $60.
If you buy the Season Pass you gain all four map packs for free. The Season Pass is $50, which saves you $10 compared to buying the map packs individually.
If you bought the Onslaught map pack individually for $15, and then bought the Season Pass, you still have to pay the full amount for the Season Pass. The discount does not work based on whether or not you have bought individual map packs.
The discount relates to buying the Season Pass instead of buying the map packs separately.
My question to you is: what have you bought? You say you bought the Season Pass....had you also bought Onslaught beforehand?