The Developer license is an addition to the normal license. The normal license allows you to install the license key on your OWN sites and the DEVELOPER license allows you to install YOUR license key on your CLIENTS’ sites.
You need to buy your own license first and then ADD a developer license; the developer license is not applicable unless you have bought your own license paying quarterly, annual or lifetime.
If you have your license then you can buy a Developer License for a one-time cost of $99.
For the full license terms and conditions click here.