I don't know if I'm misunderstanding your question, but I can't possibly see how anyone except you would be in a position to answer this, especially since you've given us zero data on which to even do some math on your behalf.

Isn't it as simple as which is the cheaper of the two ways to pay for public transportation? Can you estimate how much "using public transportation a lot" translates to in dollars of tickets? And how does that compare to what it costs to buy the transit pass pre-tax? The only tricky thing is that since paying full-fare is with post-tax dollars, you'd have to figure out your overall tax rate to know what that would be in pre-tax dollars.