Here is my dilema.

Is it possible to set the script like this:

Client A pays me a monthly/yearly recurring fee to list their link in my directory. However, paid listings receive a "Details" link that will act as a brochure for their site that includes more (custom) form fields, like address, telephone, etc. that regular listings do not get.

Client B adds a link, but does so on a reciprocal basis, and receives limited exposure.

Is it possible to accomplish this without plugins and just on an if/then basis?

I'm far from a newbie, just tired of using inferior software..cheap is expensive.

thanks.

You could determine whether the link from the initial listing leads to a detailed page or simply to the user's own URL very simply by created a field in the database with Yes|No and inserted an if statement into the templates.

If you want to determine whether the detailed page is built or not, I would suggest using the Ultranerds Plugin for this purpose.