Backlit signs or sign boxes are probably the most common form of illuminated sign and often the most cost effective.
They come in many shapes and sizes but most share a few common characteristics. An acrylic face covered with a printed graphic is placed on a metal box. Some form of lighting; either fluorescent or LED shines from behind the graphic so it can be seen at night.
Since they are so common, typical sizes are pre-fabricated which helps to keep the cost down. So if you want a standard 2' x 8' sign it's unlikely to break the bank. However, if you're looking for a custom built sign the shape of your logo be prepared to pay a little more.
The advent of LED's has allowed a little more creativity in design. The "halo" design features acrylic letters with LED's placed behind them. These results in an elegant look not available in a traditional backlit sign.