I don't know but I guess that the rate for grin is a fixed one, at
0.00675676 BTC via grin, send 2.252253333 grin
At the rate, theymos accepted higher rate than real-time price of Grin on exchanges at the moment.
As I was saying
To get a forum credit of 0.00208333 BTC via grin, send 0.694443333 grin
7$ vs 3.7$
By the time the friday price increase comes, you might buy a membership with a few cents
0.00675676
BTC ~ 23.5$
2.252253333

(grin, lol) ~ 5$
Buying now a membership with grin is cheaper than buying it with
BTC before the price increase