It depends on the guitar. If it's a good guitar with a good neck and body (like some of the Corts), then yes it can be worth it. What you're basically doing then is to bring the hardware and electronics up to the same standard as the rest. One of my two main guitars cost me R450 in '89 and I've still not found anything better to replace it with (although admittedly I've spent a shitload of money over the years upgrading it - 15 different pickups, three each of: bridges; tuners; sets of frets; MIDIguitar systems; and enough switches to sink a battleship ;D ).
It's always worth slapping in a good set of pickups - even in a cheap & nasty. Just keep the old ones to put back in if you ever want to sell, you'll never get your money back selling them in the guitar and you might be able to use them to upgrade your next guitar or at least sell them separately. Things like tuners and bridges, you can't always remove again as they usually require some sort of modification to get them to fit properly and often you can see the scars if you put the old stuff back.
The only other time it's worth it is if the guitar has some sentimental value so you'll never sell it and you just want to get it playable. (Hi Norio