How much do you actually save with taxsaver tickets?

[lola]

Hi, can anyone tell me how to calculate what the real cost of the taxsaver ticket is? I travel to Maynooth to Ashtown daily and the relevant taxsaver ticket costs €90, but since I buy it through work, it comes out of my gross pay, so the real cost is less than €90. I'm trying to work out whether it's worth my while to buy this ticket, or if it would cost less to buy weekly tickets. I'm not sure that I really get value out of the taxsaver ticket since my journey is pretty short!