What do you know about Alside windows?
It is probably the most commonly reviewed window on this site. A search would yield a bunch of info... IMO, they have a product line that ranges from bad to average (I cite the performance ratings as evidence). Most of their lines are very bulky, leak air, and have pocket sill designs which are undesirable. I've seen a fair amount of issues with quality control as well. If there is one bright spot, their customer service has always been good in my experience.
Ultimately, I'd just reiterate that your Okna quote is very reasonable on a product that is by far better than the others you've seen.