You're a bit mistaken.. A web admin is one who administers the website, maintains the server that the website is hosted on, performs CMS updates and plugin updates as they occur and makes minor changes to the design of the overall website.
A web developer/designer are the architects of the website, they will build you a site from the ground up and give you a finished working product to use as a website. They can perform the same functions as a web admin; however, have the capability of improving the site and redeveloping plugins to suit your changing needs as they occur.
If you hire a web admin, just be aware that major changes to your site cannot be performed efficiently or at all. They can ensure your site has up-to-date plugins and CMS backend material; however, when it comes to coding out new stuff, they may fall extremely short.
Prices vary between the two. I'd say a web admin with some good skills can be hired in at 30,000 a year. A web developer or designer will probably take 45,000 - 65,000 depending upon their skills and abilities and functional breadth and knowledge.
The key to getting the right one is the guy or girl who has HTML5, CSS, JQuery, JSAN, JS and PHP skills. That way, they can code just about anything you want and will be able to improve your site to make it more efficient and generate more traffic.