Unfortunately, it's almost impossible to distinguish between different web design agencies without knowledge in the field. The flaws don't show on the surface, hence the vast disparity between development costs.
Oftentimes, cheaper websites will be made using templates, with minimal customisation options available outside of colours, text and images. Most templates aren't made with speed or optimisation in mind, they're designed to look as attractive as they can to their customers.
With a Quko website, you won't need to remake your website. You'll be able to focus on growing your business, knowing that your website isn't holding you back.