I'm wondering this, too. After going through 3 portable dvd players (the batteries die but then we still use it plugged in but then the screens seem to go) ds just got a 4th for his birthday. Which didn't work so we're in the process of exchanging it (waiting for the store to get more). But they are obviously something that just don't last despite the limited use. Unless it's just us! We've had *good* ones, too - not cheapies and they don't last long so I was just going to buy a cheap one next.
Anyway, we used to have a laptop and used that, too, but I did find it kind of bulky - both in size and also in starting it up and shutting it down just to watch a dvd. I know there are some that you can attach to the back of the head rests of the front seats but I am not sure that would work with our headrests and the ones I've seen all have two screens (and are more expensive) which we don't need.
I do hope to get another laptop soon and then we can use that again. Otherwise...I don't know. Is there something like an iPad type thing but that will also play discs? Screen size doesn't matter too much to us but I understand what you're saying about the cost of a cheap laptop versus the dvd player.