2-3 years ago, buying Pro cost US$18.88 for 6 months of support & updates.
Last year (around Aug/Sept) LW.org increased the price & offered an extended version with 12 months support & updates. They do occasionally offer a cheaper version with no support or updates.
Once your 6 month support period finishes, your email is automatically removed from their databases.
http://www.gnutellaforums.com/85807-post2.html
As for notifications of your support period nearing an end (if you are a genuine Pro customer), they are reasonably frequent;
Upgrading to V 4.18