I'm not too sure. I didn't look at the stock sizes cus they didn't fit me, but I think it was something like $150 - $200 CDN extra.
It's not quite as cut and dry as that though. The nexus is available only as a stock size (only Bi-lam, soft-socks etc. as well), and the Aqua Pro is available as both.
You could get a Aqua Pro in Bi-Lam, Stock size, soft-socks, etc. (essentially identical to a nexus) and I think it would be just a bit more expensive than the nexus just because it's the Aqua Pro.
If you're looking for a basic suit in a stock size, nexus all the way. If you want any features like custom size, QuadFlex, Vulc. Boots, matrix pockets, front entry, etc. you have to go for an Aqua Pro or a Catalyst or one of their customizable suits.
IMO, it's worth it to go for the Aqua Pro just to have the QuadFlex option. I can totally tell the difference between the two materials...The QuadFlex is Way more durable.