I just can't believe how many different versions of trem pedals there are. I for one don't believe it should be an expensive effect, given the simplicity of tremolo circuits. I mean, it makes your volume go up and down real fast (or slow).
If you're handy with a soldering iron and you'd like to try a first build, this would be a great suggestion:
http://www.buildyourownclone.com/tremolo.html I've got one on my board and I like it. In fact, I like it better than the trem on my Fender tube amps. Simple, straightforward, and doesn't do the helicopter chop sound, so it behaves very much like a Fender trem.