Sure they are worth something but typically at least half of what an original board is worth. There are many many different type bootleg pacs. Some are worth more than others but they usually sell for around $40-$50. The jamma versions sell very well.
I have been sitting on a couple Gallaga bootleg boards for a while now. Not much interest in them. Its surprising because the gameplay is very accurate and its actually a more reliable board than the original was.