Unreal Engine comes with full source updated live on GitHub, that is worth its weight in gold, sadly not even the Pro Unity subscription version comes with that option.
I think that's the issue though, hard to compete with getting it all for free without any lawyers or needing to even ask, especially when you are a paying customer. It will take time but with the source for Unreal being on GitHub it has the potential of becoming an actual "open" standard of sorts given how much the community can contribute back. It also makes it a greater platform for experimentation for universities and students in addition to smaller shops that can't afford to license it. Usually when a company says that the source can be negotiated separately the price is of the form of: "if you have to ask..."
My barrier to entry with Unreal is not knowing C++ - I picked up a few books and will be diving in shortly, but I'm already good with JS so UnityScript was easy to pick up (easy, but painful none-the-less).
