From one perspective the question became irrelevant on 21 Apr 2015. That was the day Google rolled out the mobile friendly update sometimes called Mobilegedden.
For the question to be valid, your non mobile friendly site must show up in the SERP (Search Engine Result Page) of someone who conducts a Google search (and based on the specific wording of the question on a mobile device).
Effective Apr 21, 2015, non mobile friendly sites do not show up in the SERP for searches done on mobile devices, thereby ensuring the SERP to visit conversion rate is zero.
If the page is not available to be selected, it will never be selected.
Another perspective of this question is if someone on a mobile device does visit a website that is not mobile friendly, is there evidence conversions are lower than similar sites that are mobile friendly.
I know of no such specific study, and the best answer to this question (that I know of) is anecdotal.
When you try to navigate a non mobile friendly site on a small screen how does it go for you?
I find those sites fairly easy to navigate on a large screen tablet and very difficult to navigate on a small screen smart phone.
So while this is not a definitive answer, I do consider this to be evidence that even when a mobile device user does visit and navigate around on a non mobile friendly device, the conversion rate is lower, even if only a little.
But why even go there? If your website is WordPress you can fairly easily install a plugin that makes your site mobile friendly or upgrade to a mobile friendly theme.
Conversion is meaningful when your site has visitors. You attract visitors via Inbound Marketing.SEO Needs Link Building