TL;DR: There is no fixed link, because there is no need for it and it couldn’t be done right.
I am the lead developer for MultilingualPress, a plugin that does have a Pro version. We would probably benefit from such a link, but I think it is good that there is no UI for it.
There multiple reasons for its absence.
-
Buying a plugin is not a common task for some who is visiting wordpress.org. The current plugin page is already overloaded with information. Adding another tab/UI whatever would make the experience even worse. You optimize an UI always for the common tasks, not for the rare cases.
-
Most plugin vendors don’t actually sell the code. They sell updates and support.
While the Pro versions often contain additional features, their code is nowadays mostly licensed under GPL or a compatible license like MIT.
Our plugin is pure GPL. You can get a Pro version and resell it for a lower price, that’s allowed. I wouldn’t like it, but there is nothing I can do about it. Some people are doing that, some even go so far to tell their customers that they are the plugin author. That’s a violation of the license. -
There are too many different pricing models and factors: Some people sell just one license, some take one price for a bundle of plugins, some sell a subscription for the support, and there are many different time limits: from three month up to a “life time support”.
Sometimes the price depends on the currency or the country of the buyer (search for VAT europe to get an idea).
There is no catch-all UI for all these options. -
You don’t just want to ask the user to buy the product. You want to explain why they should buy it and what exactly they can expect when they are doing that. That takes some room, and you need full control over these pages.
-
Every country puts some slightly different legal requirements onto online shops. In Germany for example you need a second checkout page – that would be pure noise for someone who operates from a country without such a restriction.
-
Some vendors put great efforts into localization: That includes translation, sometimes a different theme, product descriptions, colors, typography and more.