Dinner with an Imagineer is no longer offered. Disney Dining stopped that event about two years ago. They do still offer Lunch with an Imagineer at the Brown Derby. The cost is about $65 and includes soup, salad, entree and dessert. Tax and gratuity are extra.
The lunch is held in a separate room off the main dining room. You can actually see the room if you walk past the outside dining area of the BD. You will also receive a special commemorative plate that the Imagineer will sign for you. It is a VERY LARGE plate and they used to ship it back to your hotel if you asked. Bring your camera as you will be allowed to take pictures. The whole experience lasts about 2 hours depending upon the Imagineer's schedule (they do actually have to go back to work!)
One of the special perks in the past was a special Fantasmic viewing pass like those you get with the dinner packages. My DD received one at each of her lunches and it was good for the whole family even though she was the only one at the lunch. I'm not sure if they still give these out but it wouldn't hurt to ask the manager if you don't get one.