The price of these have already dropped considerably. I have a nice little one which has a 7" screen and cost approx £90. Great for when your away on your travels.
I have a Venturer portable (which I believe cost around £70 - it was a gift). It works very well although you are aware of disc motor sound in the background. Apparently most portable DVDs have this and I've been advised it can be reduced if you place the machine on a flat surface. It can be used in the car and is lightweight to transport for travel (it has a mains connection and rechargeable battery). If you enjoy movies they are definitely worthwhile and if you buy online you will save a small fortune.