Ian,
You really have to compare cost versus quality. Z scale in general has some of the best performing locomotives in the hobby. Coreless motors, DCC ready, LED lighting and prototype specific detailing is often a key component of Z's locomotives. A GP7 in approx 3" in length.
If you really want to be amazed, visit Z scale's largest dealer at:
http://www.zscalemonster.com/
Here you will find numerous products from a wealth of Z manufacturers. Z is quickly maturing. More new and exciting releases are on the way!
Rob