I did a bit of grinding to get a creep decoration to play with on PTR. I found the process overly grindy. I could make it little less grindy by pixelizating a large batch, then converting back. But for the more rare decoration types it's grueling.
I'd much prefer a way pay a premium to pick the decoration type like we can with the theme.
Expanding from the current formula. The price appears to be
500/exclusivity * 1.43.
So randomly pixelizating a single theme costs
500/(1/7) * 1.43 = 5000.
Or pixelizating any theme except one:
500/(6/7) * 1.43 = 834.
Assuming creep decorations are 1 in a 100, pixelizating a creep decoration:
500/(1/100) * 1.43 = 71500.
Or a fire themed creep decoration:
500/(1/7 * 1/100) * 1.43 = 500500.