I'll toss in the Fischer Eterna and Klipsch Custom 3 into the mix, both great products that can be had less than $100.
On a budget, I think you are leaving out a number of great options that are not on your list. You may also look at the RE-ZERO over the RE0 as an option. It functions better without an amp and offers more dynamics, flatter response, and a little better sound stage. There are certain qualities of the note presentation of the RE0 that are a little different from the RE-ZERO that are likable, but the RE-ZERO, at least in my eyes, is a substantially better overall product, at least enough to justify the $20 price difference.
The right option all comes down to personal preference and which product out there fits your own preferences best. I tend to shy away some from suggesting the RE0 or RE-ZERO because they have heavily controlled drivers. It ends up making them a bit lean sounding, sometimes weak, as dynamic range and articulation pretty much needs to be forced out of them. The benefit is a super clean note and low distortion, but it's a trade-off. You have to want that goal to like that sound. It's s super clean, lean sound with a direct, open presentation, but at the same time dynamics and energy are not effortless. It's a sound you can't get with most other products out there, so it fits into a niche of the market but a likable niche. These earphones are popular for a reason, but the sound is a little less mainstream. I'll say the RE-ZERO is more towards mainstream than the RE0.
I was not a fan of the RE0 personally. I bought a set towards when they first came out. I was quite put off by the weak presentation. I could best describe the RE0 has sounding tiny. It's like listening to a tiny speaker playing in your ear. The presence just is very light with a very minimal dynamic range. This is why people suggest pairing them with a robust amp. Even a mild amp like the FiiO E5 isn't really enough for them. They really benefit from a powerful amp. The RE-ZERO offers a better presentation right out of the box, not having that tiny sound, and it works great off portable devices without any real amp need. I actually like the RE-ZERO and see it as a good earphone. I do see things like the Eterna and Custom 3 offering a little better presentation overall though.
I'll just say this. The best sub $100 earphone I've used is the Custom 3. Even if I was looking at $200 to $300 products, the Custom 3 is still there showing them a thing or two.