Red Runes are roughly every 20k points gained in 8 Gates, which I suppose adds up to about 50m more or less (I actually forget). But as always that's not absolute.

On my first 100 million dumped into gates at 65 I got a ton of golds and I think 2 Reds, on my next 50 million dumped in a few levels later, I got nothing noteworthy, then on my most recent 50 million I got a bunch of useful golds and a red (Which I sold).

So in my experience I feel like Gates only feels worthwhile when spending at least 50 million or more.