Sare when you write a proggy for someone, you're doing a custom, one-time job. You're not going to turn around and sell that same script to someone else - you'd write them a new one.
You *can* compete with the php guys out here, you just have to think differently. Sure, if you pay yourself for your time, you'll have put about $500 into writing the script.
But since it's not written for a specific client, you can turn around and sell it 10 times for $75 each and be making a profit already. Keep pushing it, and sell it 50 times... $3750 for something that took you 5 hours to do.
Not bad for part-time work, eh?
Plus the more complex the script, the more you can charge. You'll want to structure anything you write as a modular thing, so that there can be different levels to the script and things can easily be added. A basic script could be $50, but add in the link-checking module and the hand-submit (write the word you see in the box) module and it's $75.
Want the full kabang that does everything but wipe your keyboard off when its finished? Then you're talking a few hundred dollars or even a thousand dollars like the High End Triple or the AVS Matrix. (I would never pay that much for a script, but lots of non-programmers would!)
The great thing about your scripts is that you write it once and it's done. You can resell that sucker as many times as you want without putting another minute of work into it. Unlike Jen's jewels... when she sells one, she has to sit and repeat all of the time and effort to make another. Scripts have very low overhead and absolutely no space needed for inventory...

It's just controlling yourself and following your original plan that is the hardest thing. My problem is I keep adding shit on and getting more ideas of stuff it can do when I'm halfway through it. (That's also why plug-in modules are so essential... super-easy expansion later.)
Custom programming is a good way to make a was of cash in a short amount of time, but writing a resaleable script - it's like the gift that keeps giving!