There is a fine balance between breat UX and clever UI. A beautiful looking website might get top marks in UI, it looks great and people ooh and ahh. BUT when they start to use the website to achieve their goals quickly and efficiently they may start to get frustrated.
It takes me three clicks to get to the page I want to yet the competitors page only takes one.
Its true, although the site looks great using it becomes frustrating after a while.
Conversley, another site may work really well but look like a mess. Obviously a balnace between the two is crucial and the balance can be lopsided depending on the complexities and requirements of the brief.

A single proposition website, for instance selling one product or range of products for a good price in a high demand industry, will sell products almost by default.
However if your website represents your company in the same way as a sales person for instance then your site needs to look pretty good. No one would send a sales person out in scruffy suit and dirty shoes.
Today your website is part of your overall brand package. It is often the first impression your company makes. How many times have you been on a company website looking for something and left frustrated and unlikey to ever return. Many good long term customers are lost in this way.
I would suggest you should spend on your website equivalent to how you want to be percieved by your customers.