VIP9 -- I just went through the same searching you did, exploring all the alternatives (convenience, cost, etc) from drop downs, to bag systems, to portable DVD players. Here are the alternatives I came up with and what I am doing.
Bag systems. You can get an Audiovox VCP bag system with a 4" or 5" screen for around $229-$299. Sears or Best Buy carries them. Funai is another mfgr which can be bought at Best Buy for $250 for 4" screen. Pros: Cheaper. Most older titles (and kids shows) are in VHS format. Not permanent. Cons: Unsightly (mounted with straps). Have to remove it or hide it to keep from being stolen. Smaller screen. Wired headphones. Poor visibilty from 3rd row seats.
Portable DVDs. Prices range from $499 to $1299 for a 5" to 8" screen. More $ gets a bigger screen. Yahoo Shopping is a good price source, sometimes you can save up to 20-40% off direct retail. Bought a Toshiba 8" DVD SPP1500 for $813. Retail at Best Buy was $999 at the time. Pros: Picture quality and screen size. Portability outside of the car. Cons: Costlier. Issue of mounting and safety (although there is a thread on some who either strap or velcro to the console lid). Less visibilty from 3rd row seats.
Drop down. Prices ranges from $1200 to $1900 installed for a 5 to 7" with VCP or DVD (slightly higher cost). Mfgs are Audiovox, Accelevision, Alpine, Jensen, Pioneer, Clarion. Some come with TV tuners (to get local broadcast tv), wireless headphones, remotes, etc. Some come in packages (Audiovox is one), or you can cusotmize with individual components (DVD vs. VCP). Pros: Clean looking fit when professionally installed. Little setup, always available for the kids for the quick (30-45 minute trips). Safe installation. Better visibilty from 3rd row seats. Cons: Permanent (although I have been told you can remove it and patch the mounting holes). Costlier system.
A note on Audiovox - Many people I know and retailers I spoke with (even some who do not carry the product line!) actually speak highly of Audiovox. They claim the screen is clearer and on a cost basis, the best value for the money. Audiovox's history dates from installing tvs in RVs and have significant info on mobile products. In my comparison, Audiovox was the best against comparably priced mfgrs products.
My solution - I purchased from Yahoo an Audiovox pacakged system (MVP300D) with 6.8" Drop down screen (non-tuner, VOH681), DVD player (MVDVD4), FM modulator (to play thru the stereo, FMM100) and 2 wireless headphones (MVIRHS) for around $850, including shipping and NO TAX. Retail on this same system at Circuit City was $1300, excluding tax and no installation. I will have it installed which will cost me $300. I am in for $1100 versus the $1600 to $1800 I have been quoted for the same system elsewhere.
I liked the fact that I did not have to deal with installing the thing each time (although I know I can hide the DVD in the console) when I take my kids across town. It is out of the way (not in the footwell for the bag or console for the dvd). It looks like it belongs in a $42K+ vehicle. I do not have Barney on DVD!
I welcome any additonal questions.