I think you could ask for a percentage. Lets say the album costs 20 US Dollars, consists of 10 songs, of which 1 is yours. So your song is worth 2 US Dollars. Because they sell your song as their own work, I think you can ask for lets say 30% of the benefit of the one song that comes from you.

With the above calculation that would be something about 60 cents for each album that has been sold. If you go up to 50% of the benefit, you would get 1 Dollar for each album... But don't forget, you just wrote the song, while they have the costs for producing, recording, publishing burning etc. Therefore I definitely wouldn't ask more than 30-40% of the benefit of your song.