A set of gels and a holder is about $30 total. I use a LumiQuest holder and Rosco gels.
The process is simple: put a gel over one strobe. Set it to full power, and position it on the floor so that the light goes straight up the wall. Adjust your aperture so that you get the intensity of the background you want.
Next, set up your second strobe as a model light. Adjust the power of this strobe until it matches the exposure you've already set for your background.
With just a few minutes of effort, you can get a variety of color backgrounds to suit your shoot.
The power of the strobe is much more than you might think. Below is an example of a gray wall in the corner of a room that was made white with a strobe placed on the floor.
|A plain wall about eight feet wide.|
|The same gray wall, illuminated with a strobe on full power, turning it white.|
By pulling your subject away from the background and using only a single light, you can also get a background pretty close to black, as below.
|Experiment with separating your subject from the background and using a lower power setting.|
You can get a variety of background colors with the gels. These are iPhone pix of the camera's LCD screen as I experimented quickly changing the gray wall to a few different colors, using a doll as a model.
Depending on where you place the "background strobe," and how wide you zoom the head, you can also vary the effects of the light fall-off, making a more textured background. The only cost to experimentation is your time, and recharging the batteries. Have at it!