We had similar conclusions: the free version of Go, the "$499 for annual support on the free version", and the "$499 for 3 remote agents" were worth considering, but the price jump after that (for the "enterprise" version with release management support) was astronomical.
For commercial alternatives, there's Electric Cloud, but it's even more expensive than the ThoughtWorks product.
For Open Source, there's Cruise Control (Go is a re-written private version of the open source Cruise Control), Continuum, LuntBuild, Anthill, etc. These are mostly continuous integration build systems (i.e. more emphasis placed on running the build than managing the results) but they do seem to work well (I've used Cruise Control and Hudson in the past with much success.)