I wouldn't buy something just because it has a tube in it. A cheap starved plate tube preamp like the ART or Presonus stuff is very different from say...a Groove Tubes Vipre.
The least expensive, non-starved plate, tube preamp would be the Groove Tubes Brick although it doesn't have a really tube-y sound nor does it offer a whole lot of gain.
The ART MPA Digital is not starve plate designed, and I've heard a lot of good things about it. It retails for 300$ I think.