I remember coming across a blog post for a company that built some of the first rack mount cases for the latest Mac Pro (the trash can one), that had a similar hosted service to thumber. I'll link to it if I can find it.
Their earlier article about racking mac minis is good too: http://photos.imgix.com/building-a-graphics-card-for-the-int...
For example, you can get a Google Cloud instance with 32 vCPUs and 28.8GB of memory for around $622 a month.