I know this has been done a million times but I made my lightbox today and it is awesome.
I had the lights already and they were about $5 per fixture and $6 for 4 bulbs. I am using the spiral screw in flourescent and the light and color is better than when I use standard incandescent bulbs. The foam board was $1.99 per sheet and since I did not have white masking tape I got a roll of 2" tape for about $2. I added an extension cord with 4 outlets for $6.
In total this cost me $44 and if wanted to save money I could have just used standard bulbs I had and extension cords and some artists tape. The down and dirty version would have been $30. Not too bad.
Here is the lightbox. Samples to follow.