There isn't an easy or even a good answer to your softbox question...
In terms of value for money, basically you get what you pay for. There are plenty of cheap softboxes around but generally they're just knock-offs of good makes and the people who make them know absolutely nothing about lighting and don't seem to understand why softboxes need to be designed well and made from the right materials. Typically, the inner diffuser (if it has one) will be far too close to the front diffuser and there will be a big gap between the edges of the diffuser and the walls of the softbox, so the light will be pretty uneven.
Typically, both the inner and outer diffuser will be made of ridiculously thin (and cheap) material and it won't diffuse the light properly.
And although it will work, the build quality doesn't normally allow cheap softboxes to be taken apart easily, or very often, if you can't leave them up permanently.
As an experiment I bought one of these cheap softboxes from Ebay a few years ago and I was able to make it usable, see
this link.
But moving the diffuser and doubling the thickness of the front diffuser didn't alter the fact that the front diffuser... is at the front. The better softboxes have the front diffuser recessed a couple of inches, this does a couple of things, as well as allowing a honeycomb to be fitted to it.
1. with the diffuser recessed, the softbox can be used to feather the light - just catching the subject with the edge of it or using it to produce graduated light, say on the background.
2. Without this recess, the light goes just about everywhere and pretty well forces the softbox to be behind the camera, to avoid lens flare. The recess allows it to be more versatile and allows more creative use of the light.
Size? Well, that depends on style and space. Ideally you should have loads of different sizes and shapes for different purposes, but if you can only get one it makes sense to get a big one, because a big one can produce soft light if required and you can always change the shape by masking the front with blackwrap. But big softboxes take up a lot of space if you have a small room...
One of the best selling Lencarta softboxes is the 140 x 30 cm strip softbox (in both folding and non folding versions) OK, it's perfect for what it's designed to do, provide a narrow strip of light used vertically, but a lot of people use them for groups too, using them horizontally and high to produce a big softbox effect across the width of the group, with the light fading as it gets lower - useful in any studio but perfect where space, and a low ceiling, doesn't allow a really big softbox to be used.
