If it helps, I got the cushion brellia recently myself and feel that its definitely worth it. It's extremely firey.
I already one already reserved and in the designing process. Got exactly what I wanted and a beautiful touch they have done for me was to add cushion brellia melee diamonds which I didn't know was possible.
The custom cut melees are more expensive than the rounds too but I figured that the cushion has such large fire, it's worth it to bling up the entire ring.
Overall, I'm extremely impressed with the entire process. My SO loves the cushion and their suggested design exceeded her expectations. This is the current superimpose they did on her hand photo.
About the pricing, I was told there is more carat wastage incurred when cutting the brellia. So you are right there is about a 5-10% difference in the price compared to the round. But very worth it if your gf wants the best performing cushion.
This is my cushion's scopes
P.S. If you guys want to know, I was served by a lad named Jerome. He is very friendly and no hard selling at all. He knows his diamond stuff well and I'd recommend anyone to look for him.