Like someone said before.... 10% more. No idea what rune mining is like for cash (always prefered slaying/bossing) BUT say it's about 400k/h, you should be making 40k/h more.
If you can afford the Dragon Pickaxe, then I say go for it. You get about 8K-10K more xp than you would with a Rune Pickaxe per hour. I guess mining with a Rune Pickaxe would get you 99 about 3 days later than using a Dragon Pick.