Raiden Fighters Aces. It's a great shmup compilation that has Raiden Fighters, Raiden Fighters 2 and Raiden Fighters Jet, and it costs under $20. Plenty of ships to choose from in each game, the games have good depth in their scoring techniques, and they offer a solid challenge.
Raiden IV's price is slowly dropping (around $30 now), but I wouldn't call it cheap just yet. It too is a good game, with a nice mix of oldschool gameplay, and modern graphics. It also comes with an enjoyable music CD featuring the soundtrack of the game, as well as remixes of songs from the franchise.