Beyond a certain level, price is a relatively poor metric of device performance, besides it also depends on some unrelated factors like economy of scale (the more of something you can make, the more cost-effective it becomes). An amplifier has to get the job done, i.e. approximate a "wire with gain" at the SPLs required. If the frequency response is sufficiently flat, you can't hear any noise or distortion and you never hit its power limits at your listening levels, that's good enough. You can spend more for better parts quality or improvements in other parameters just for the feelgood factor, but is it going to sound any better? Nope.
For many folks with typical bookshelves in a near to midfield setup and without extreme level needs, the point of diminishing return may be a Yamaha A-S301 or a Topping PA5 (ca. $350 - you can get away substantially cheaper still on the used market). Amplifiers are at a high enough level that a half-decent one will generally perform fine. The spread among speakers is way, way greater. Finding two amplifiers that sound the same under normal circumstances is not a truly major challenge, but try the same with speakers and it's a much different story.
