Actually in the sciences, most good universities give stipends to the PhD students and waive tuition. Thus is due to grant funding, the NIH, HHMI and the NSF grants pay overhead on grants to cover the cost of PostDocs and grad students, which includes tuition to the university. So, if the program is in a university that is well stocked with grant funded faculty, the student is almost absolutely not paying tuition. It is VERY different outside of the sciences. University if Colorado is well funded and respected, so there was almost no chance that he was paying anything, and he probably was getting around 20k a year in stipend to go there.
PS, you all think this is so inviting that you want to try it? Well, they work like slave labor for 20k for around five to six years, then get a 2-5 year job as a Post Doc for a whopping 30-40k a year and still, as an academic researcher they are lucky to hit six figures in 10 years at the age of fifty. Sound good? Compare this to a 2 year MBA, make 6 figures in a couple of years. Or to an MD or JD, less time in school for them and much larger salaries with no Post Doc years. Everyone I know that goes through it doesn't do it for the money, as we all know it isn't worth it on that end.