Let's say I create a CMS that uses ExtJS.
I want to avoid the license fee, so I open-source the CMS on github.
Now let's imagine that I make your friend Dave a website that uses my CMS. I spend three hundred hours designing a logo and layout. Can I charge a fee for this, and would I be obliged to open-source Dave's website too or is it enough to just open-source the CMS?
I find this LPGL license a little confusing. Say hi to Dave for me.
Only ExtJS code prior to 2.1 is under LGPL. It currently uses GPLv3. That means if you distribute it, "the entire work, as a whole" must be licensed under GPLv3. The entire work may include code directly connected to ExJS. But "separate and independent works, which are not by their nature extensions of the covered work" (which probably includes server code) don't have to be GPLv3. So the way I interpret it, only code (e.g. client JS you develop) that tightly extends Ext would have to be released. See also this FAQ.
Regardless, you can charge as much as you want for services and/or the CMS itself.
IANAL.