They tend to teach the art and technique of filmmaking rather than the practicalities of planning and budgeting to meet specific audience and market expectations. There can also be a lot of hype and marketing BS in this area too. The classic example has already been mentioned, "El Mariachi" which was made with a budget of $7k. What isn't as widely publicised is that the final version the public got to see needed a further $300k spent on it to give it "a professional look" sufficient to meet commercial expectations!
I think it's more than just hype & marketing BS though - in fact it's a perfect lesson in the practicalities of planning for "specific audience and market expectations" if you look at the $7,000 figure in the context of his goals.
Rodriguez never intended for El Mariachi to see a widespread U.S. theatrical release. His plan from the start was to sell it to a distributor in the mexican home video market for around $25k and then use the money from that to fund the sequel, repeat for a third one and then shoot for a hollywood career based on the experience gained from the three films. The fact that it got studio interest at all was mostly luck and good timing, and the additional cost was to prep it for something it wasn't meant for in the first place.
For instance, he shot with no sync sound... thousands of dollars were spent later on a professional audio engineer working in a studio for weeks trying to re-sync dialogue to the print (who finally gave up and Rodriguez finished the re-sync himself). Now obviously the 'better' solution would have been to shoot sync in the first place - but that would have required a different camera, audio equipment, a sound guy, possibly even different locations, and would have slowed the whole shoot down. If he'd tried to do it the right way for a theatrical release it would have significantly impacted the budget and his ability to get the film shot - but he wasn't going for that, so he was able to work more efficiently based on the resources he had available at the time and the particular goals of the project.
Thus the fact that he was able to make the initial film for $7,000 was the result of exactly what you are suggesting - being honest about his goals and spending no more than necessary to achieve them. It's entirely possible that if he'd set out in the first place to make the next big indie action film it wouldn't have happened at all, and we wouldn't be talking about him now.
It's far easier now from a technical standpoint to produce a film for $7,000 and achieve something closer to a commercial release quality than it was back then. The $550/week rental package I listed earlier in the thread will get you better equipment than Rodriguez had when he made mariachi, and the cheap post production software available to anyone with a reasonably modern computer will get you far better results than the post resources he had available to him at the time. The part that hasn't become any easier is the need to hustle to get the most out of whatever resources you can lay your hands on, and then hustle to get the project done.