Over the course of acquiring over 250k practice points i find that on average you get a red rune every 40 - 50 million coins spent. But that's an average over a very long time. Just yesterday i had to spend 68 million coins before i got a red rune, and a few weeks ago i went over 75 million before a red rune. On two separate occasions i got red runes less then 5 million coins apart. So in small sample sizes it is very RNG dependent.

I do feel that they lowered the rate of gold runes awhile ago though. When the system first came out I really feel like i was getting a lot more gold runes then I do now, but i have nothing but my gut feeling to back that up.