I fully agree, increasing a conversion rate from by 0.5% on a site that earns 10k a year with a 1% CR brings in an extra 5k, and all it costs you is your time tweaking the existing site, rarther than having to bring in hundreds or thousands of new visitors.
The knock on effect also allows you to be able to bid higher in Adwords and gain more visitors that way, as each per visit value is higher