Two pairs of sunglasses costs 2 x $50 = $<<2*50=100>>100.
So, Mary spent $100 + $100 = $<<100+100=200>>200.
Thus, two decks of basketball cards costs $200 - $150 = $<<200-150=50>>50.
Therefore, one deck of basketball cards costs $50/2 = $<<50/2=25>>25.
#### 25