It can be very difficult to sell usability, as Jared Spool notes in this 2004 interview:

I learned quickly that business executives didn't care about usability testing or information design. Explaining the importance of these areas didn't get us any more work. Instead, when we're in front of executives, we quickly learned to talk about only five things:

1. How do we increase revenue?
2. How do we reduce expenses?
3. How do we bring in more customers?
4. How do we get more business out of each existing customer?
5. How do we increase shareholder value?

Notice that the words 'design', 'usability', or 'navigation' never appear in these questions. We found, early on, that the less we talked about usability or design, the bigger our projects got. Today, I'm writing a proposal for a $470,000 project where the word 'usability' isn't mentioned once in the proposal.

If you can't talk about usability, what do you talk about? How many customers you're losing. In the internet era, the cost of switching to a competitor is approaching zero. If your site is more difficult to use than your competition's, you're in trouble. A recent A List Apart article highlights the dramatic results of a usability overhaul:

We tested a subscription route that [..] asked for a lot less information on the subscription page.

The outcome? An increase in conversion rates of over 500%. That is to say, of the people who arrived at the subscription offer page, we increased the number who actually signed up by over 500%.

And while we sold a little harder -- and offered an incentive at the back end -- the primary cause of the increase was almost certainly that we reduced the friction during the sign-up process. We offered more and asked for less.

With another partner we cut back on the number of pages involved in signing up for a paid subscription service from nine to three. (Yes, a nine-page process was more than excessive.) The result? An increase in sign-ups of 293%.

You may have trouble selling usability improvements-- but I don't think you'd have any trouble selling a three to five fold increase in sign-up rates.