To change my front tires, do I really need to pay $100 for alignment as well?

If your tires were badly worn then you should have the alignment performed. $100 is a typical price for all four wheels in the bay area.

I always reccomend that the alignment be checked any time tires are replaced. Tires are costly and a alignment being out can cost you extra wear in the tires and cost more fuel to be used. We charge $80 here on the east coast and the price you say is probbly in range for the left coast.

If the tires seem to wear unevenly (say the inside has no tread but the rest of the tire looks decent or vice versa).... then it is highly recommended to get an alignment done or the new tires may have the same fate. You want to get the most value out of your tires.