How much does it cost to make a site mobile friendly?
 

I have an olllld site that looks like shit on a phone. How much does it cost to make it mobile friendly?

is it just a matter of some simple css adjustments with media queries or is it a major overhaul of everything?
just a 20 page site that has 3 columns
just a ballpark figure

What's the URL?

Depends on the site. Relevant factors are how many things you want to be responsive, how many different resolutions it should change at, and how modular the HTML and CSS are.

Potential obstacles are fixed image sizes that are part of the design.

Post the URL and what you want to be responsive and we can give you a better idea.

It might be tricky to tack on responsive design to an already-finished desktop page. Most responsive pages were intended to be responsive from the moment someone started coding.

From what I've seen, most sites that had desktop versions before mobile took off are using subdomains (m.xyz.com) instead of responsive design.

That is one way then use <link rel="canonical" href="http://xyz.com/" /> on the mobile subdomain.