Nitrogen is technically better. Pragmatically, that still shouldn't
Mixing the two isn't a problem, but reduces the oxidation technical benefit. Topping up nitrogen with air isn't adding much oxygen anyway.
If you have money to burn and don't mind being scammed, sure 40$ (or even 4x40$). If you want what's technically better (I usually do), find a reasonable price elsewhere. Otherwise, cash is better used elsewhere in life, or on directly-useful automotive products (rust inhibitor, new wipers, better oil or more frequent changes, etc.)