There was most certainly a discount for Plus members on I Am Alive for approximately half-off the full price. Unfortunately, the deal was only available through the first week of the Christmas sale. The entire slate has been wiped clean this week for a whole new slew of gaming options you may or may not have already played.
The best chance a sale like that will return may come up again around January or February (that's usually the time when the PlayStation Store has a user-voted sale containing multiple selections. Either that or the chance that Plus members may get the game for free somewhere down the line is a possibility as well.