Getting admin to work on a "dev-sounding" subdomain
As Luke referenced in his answer, you're mixing up two separate (but related) concepts.

Craft License

Your license key is stored in your craft/config/license.key file. This file is automatically generated the first time you run Craft, and should always remain a part of your website. If you're using version control, make sure that it's included in your repo.

Craft Edition

The "edition" of your website refers to which version you're using: Free, Client, or Pro.

Whichever edition you are currently using will be stored in your database for immediate reference... However, your site will frequently ping Pixel &amp; Tonic's servers to ensure that it's using the correct edition. While your license key does not directly contain which edition you should be using, it let's P&amp;T's servers know which website is performing the request, and replies back with the proper edition information.

How it relates to your situation...

Using a "dev-sounding" domain name only does one thing:


You can share the same license key file between domains without getting a warning message at the top of each page.


The P&amp;T servers will associate your license with only the non-dev-sounding domain... any dev-sounding domains will not be recording with your license key data. So if you try to use the same license key file on multiple non-dev-sounding domains, all but one will display this message:


  The license located at config/license.key belongs to [OTHER DOMAIN]. Transfer it to this domain?


In regards to your edition... As long as they're sharing the same license, every environment for your site will share the same edition.

As Luke mentioned, when developing a site in your local environment, you can set the local path to end with craft.dev. This is considered a very special circumstance... Craft will recognize this special domain and allow you to upgrade your edition for free!

Of course, since that special domain is only available locally, you'll be expected to properly purchase your edition when you move your site to any other domain name (regardless of whether it's "dev-sounding").

Again, Luke's solution is correct... Purchase the license now, and don't worry about what the final domain will be. It's not relevant. Switching your license to the correct domain is incredibly easy and painless, and happens with a single click.

UPDATE:

It's now possible to try Craft Pro from any non-public domain:


  http://pixelandtonic.com/blog/test-craft-client-pro-from-any-non-public-domain