I don't agree and I'll explain why. Maybe it will look clear at the end (please, take some time to answer, don't just reply to offend my uncommon opinion)
Let's take out of the equation some very expensive parts, and let's take into consideration gun parts which may cost from 5 to 200$
A very easy to understand example for what I know :
Cz Shadow2, ipsc production rules.
Stock shadow2 vs main spring 11LB.
In the first case, to do what you can do with the 11Lb spring, you will need so much practice that it could literally be a "waste" of money and time.
Let's say it's 20$ in the worst case. You spent it once, and here you go. 100% sure it will take less time to press the trigger, OR it will mean less mistakes done due to pressing it. Have you ever considered how much points you might lose (or not gain) due to using a bad gun?
Yes, everybody can have good results with a bad/average gun, with a lot of practice. Is it smart? I can bet it's not.
It's the same example you can use for training courses or practice.
Another part is : HOW MUCH will that part change your results?
To me, for example, the RIGHT recoil spring will give HUGE results compared to the effort, while practicing with a BAD SPRING and trying to have the same results will take MUCH MORE EFFORT.
After some time, there will be a stable point, so, trying new parts will probably give 0 to little improvements.
The key measurement is improvement/cost. If you are decent at analysis, you will probably find out what is more cost effective.
Another simple example for what I know :
1500$ to spend:
Let's say it's 500$ for the Glock and 1000$ for the Shadow2.
Saying you have 500$ left to spend on practice is a really bad idea to me. It won't ever be enough to cover the disadvantages of using a Glock. Maybe you will have to shoot 30'000 rounds to be somewhere near the same level of a 1000-rounds shadow2.