My thinking is Sencha and EXT.Net tow different company and EXT.Net use a commercial product of Sencha to produce their(EXT.Net's) own product.Then why EXT.Net's price and license conditions depends on Sencha?after all 3500 USD is to high for many developers.
Ext.NET and Sencha are two different and independent companies. Ext.NET is the official .NET OEM Partner for Sencha. This gives us rights to bundle the Sencha Ext JS framework with Ext.NET and sell under a commercial license.

For each Ext.NET license we sell, a portion of the sales fee is paid to Sencha.

Sencha has decided to no longer offer single developer Ext JS licenses, and because of that we no longer have the option to bundle (resell) in Ext.NET.