I'm still using my 9mm blanks, they might cost more than .209s but they're a long way from being expensive. I might go back to 209s because I still have over 70 that I hardly use. There isn't much difference between the two really bit the 9mms are significantly louder, especially in small spaces.
I found 209s were 5p a throw and 9mms were 40p a throw (£5 for 100 and £20 for 50 respectively) it's a big difference but neither one is expensive compared to single use pyrotechnics which are often around £3 a throw