You shouldnt need to pay w3c anything to get an error free result. It should be part of your site QA/QC process; you just need to keep running it and correcting errors; it takes time but it should be done anyways. Error free clean code with good markup is just a good best practice all around.
And if you dont do it for SEO think of it this way; you have a wordpress blog. After a few years you are seeing 25,000+ visitors per month because of this you have migrated from a shared hosting plan to VPS to dedicated server in conjunction with a CDN to handle the traffic load. Maybe you see the potential of it growing even more; so you invest on migrating the site to a cloud environment. These moves become so much easier having clean semantic code because you dont have to worry about the theme having to be hacked together during every move to make the site actually work.
Or how about this reason - wordpress releases a new version; suddenly all the hack code you've used to be browser compatible (and was error ridden in w3c) with your theme now makes you theme no longer work. Now all of that hard work you've dumped into making your website look, feel and act how you want - is now all gone.
Be the SEO person who does things right and do right by your clients; it will take more time, it will be more …