Is it worth getting a suspension seat post as a cheap alternative to full suspension bike?
You might find a suspension seatpost to be comfortable, but a suspension seatpost is not a substitute for a full suspension bike.  The purpose of a suspension seatpost is purely comfort (though as mikes mentions, not everyone agrees that they achieve this goal), where as the purpose of a full suspension bike is first and foremost control.  Suspension on the wheels helps them stay on the ground over rough terrain, which a suspension seatpost will not do.  Any extra "comfort" is simply a side benefit.

If you're looking to smooth out your ride, the #1 thing you can do is lower your tire pressure.  You will lose some efficiency when the ground is smooth, but if you're already riding 145 on a bike with a front suspension, I suspect efficiency isn't your top concern to begin with.