@BitBustah. I reckon not very high. I would speculate that it is less than 100 copper memberships paid for in grin hehehe. It would be easier to pay for it in the cryptocoin they already hold.
I can't find the post now but I think theymos mentioned that it was 5 in 2 months. It's been ~4 months now so perhaps it's up to 10

. I was considering paying with grin for my bot account but it was just too much hassle to bring up my semi-cold wallet... ended up paying with BTC.
Edit - here it is:
Since grin started being accepted, 5 out of 687 payments (0.7%) were via grin.