More expensive projectors have all sorts of reasons for being more expensive and IMO people often buy a brand name (ie: Runco) instead of doing some research first to actually get a better projector, like JVC.
The Panasonic is a solid projector and is one of many - FYI, for the money, I would go Epson, not Panasonic, but I digress... The AE4000U is a solid unit with good image quality and excellence in many areas.
But, the JVC is marginally superior in almost all areas which will allow it to perform and look better under ideal conditions. Under less than ideal conditions (a light colored room) the JVC won't produce one bit of noticable difference compared to the Panny.
The biggest difference is in the capability of the JVC units to produce some of the best black levels on the market for any projector currently in production. A native contrast ratio of that type hasn't been matched by other manufacturers and creates a phenomenal setup.
But, the cheaper JVC models do very well also and are worth considering for sure.
Yet, if your room is not ideal, or you just don't have the money, then a projector like the AE4000 or 8500UB are great ways to go.
There are other more expensive models out there from companies who can handle really large screens (Digital Projection) with ease, or those with interchangable lenses which allow for shorter/longer throw distances.
If I had a serious dedicated home theater, the $5000 range would be where I would look and a JVC would likely be at the top of my list. Under $3,000 I would get the Epson.
Under $2,000 I would probably go with the Mits HC3800 or really save some cash by going with the Optoma HD20.
It can be similar with speakers as you spend more or other audio gear. Spending $1,000 on a projector gets you 80% of the way there. Spending more for the Panasonic or Epson adds another 10% pretty easily. But, that final 10% can really start costing you some serious coin.