I would probably go with K2 of the three, simply because K2 is better known for their bindings.
I think you can get better bindings for under $100. You just have to look around. What size?
Here are some better bindings for under $100:
Ride VXn Snowboard Bindings - Women's at REI.com
Ride LXh Snowboard Bindings - Women's - '09/'10 at REI.com
Ride DVa Snowboard Bindings - Women's at REI.com
Ride LXh Womens Snowboard Bindings Reviews & Sale | SierraSnowboard.com