You do get what you pay for and it should be that way. Imagine if you did not get what you paid for, there would be know way of knowing what quality of item you are buying. In a capitalist system, the amount that something costs should determine the quality of the product.
Yes, I think that you generally get your money's worth over time. If you pay more for a car, you generally get a higher quality interior, more gadgets, and a longer lasting engine. If you pay more for a meal, you get a more glamorous experience and (hopefully) a sumptuous plate as well.
I believe there are some things that the saying, you get what you pay for, applies to. Generally, we have tons of products in America that aren't worth the asking price and when that price is lower than expected you can bet the quality of the product is too. Clothing can sometimes fit this motto, cooking ware usually, and a plethora of other products.
In order for this saying to be true; a product that costs twice as much would have to be double the quality.
For example; if I have 2 bottles of hand sanitizer: one is 1$, the other is 2$. Both kill 99.99% of germs. Now for this idiom to be true the 2$ bottle would have to kill 199.98% of germs. Twice the price; twice the quality. You get what you pay for?
Now there are definitely many instances where price does effect quality, take a phone for instance. If you spend 400$ on a smart phone, an 800$ smartphone will most assuredly be a better device. However, in almost all cases you still will not be getting what you pay for. In order for the saying to be true the 800$ phone would have to have twice the performance and build quality of the 400$ phone.
This is where diminishing returns come in, at a 400$ price point you're getting a pretty solid phone. It's not plausible to make one twice as good; maybe even impossible with current technology. We say this idiom all the time but logic proves it simply implausible as a phrase to use as fact to determine an objects true quality.
A lot of the time it can be true when we're talking strictly from a bill of materials standpoint. When comparing apples to apples however, the bigger brand will always be more expensive. This doesn't guarantee that it will be higher quality or more effective. This is especially true with electronics. I advise everyone to shop around and buy from smaller manufacturers when it makes sense.
I don't believe that we get what we pay for all the time. This would mean that nothing in the resale market is worthy. For example, we can find a wood coffee table for new for $400, but we can find one used for $100 or less quite often. It could be the same table and we'd get the same use out of it. So many people are scared of buying used though.
While it is true for the most part, the term "you get what you pay for" isn't always factual. I have at times in my life paid for items and services that far exceeded my expecations and hopes despite te low price I may have paid. I think people just need to be smarter about what they pay for.