Re: Which Softbox?
First of all, the first two images are umbrellas and the last one is a softbox that is made for a Paul C Buff (Alien Bees) studio light, so it would have to be modified to use it with a portable flash. I wouldn't suggest you go with it this.
If you have never used a light modifier before, I would agree with the other posters and suggest you try getting started with an umbrella. The first listing is probably going to give you the most flexibility to start. You can use it either as a conventional reflective umbrella or remove the cover and use it as a shoot-through umbrella.
In order to complete your kit, you will also need a stand and a clamp (with shoe) that allows you to mount your flash and umbrella to the light stand. My guess is that you will have to almost double your budget of $50 to put together a basic modifier.
If you have not used a light modifier before, I would suggest the umbrella would be the best starting point. Softboxes are a tad trickier to use and I've only used them with studio flashes, never with Speedlite type flashes. With their larger spill, you can be off a bit more than with softboxes, and frankly, they are a bit easier to set up and use. The secret with both types is to have them positioned fairly close to your model. If you are using an umbrella, the distance to your model should not be a lot more than the diameter of the umbrella. Get it too far away, and it will start acting like a point source and the light will be a lot harsher and will cast shadows.
You should also look at investing in a white reflector. I use a piece of coreplast (hollow plastic) that is about the same diameter as the umbrella or a piece of bristol board bounce some light back at your model.