We recently moved to GitHub Enterprise and it really is not as expensive as it looks. It is $250/year per developer... not really that much considering what you get. Plus it is a virtual server provided for us, so we just have to upload software upgrade packages to get the latest updates. We didn't have to spend the money on someone to set up all of the MySql, Redis, Solr, etc. services to get it running. So honestly I consider GitHub Enterprise to be quite a cost savings. Just a counterpoint...

EDIT: We are not a startup, but still something to consider.