Consider this: if you are in the business of providing web development services or in the web design business, you’re likely to have built several applications and data intensive websites over the years. Almost all of these websites/web applications can benefit from qualitative upgrades using Ajax techniques.
For a reasonable charge, boring data entry and retreival applications can be souped up using any number of easily integrated visual interface elements. You are not messing with the internals of an application, just improving the user experience so you’re unlikely to hit major roadblocks. The resulting ‘Ajaxed’ application looks and works much better, the client looks great to their management and its a win situation for everyone.
The easiest way to sell this is to invest the time in re-engineering say one web page/report (with some nice visualization effects) and letting the client see the benefit for themselves.