So here are some things I am looking for
- Under $150
- Good noise isolation
- More or less neutral sound
- Will be used without an amp
I guess from my perspective it depends on which, if any, in-ear monitor you're accustomed to at present and what other headphones you own. I have owned 2 pairs of Ultimate Ears Triple Fi 10s and I'd struggle to live with anything with a lower level of sound quality. The new ones are a bit above your budget though. However, if you would consider buying used, I see Ultimate Ears Triple Fi 10s on Amazon.com at $150.
In my opinion they're a fairly neutral in-ear monitor that offer solid isolation provided you find the right tips for your ears. There are a good selection of tips available so you should find something to suit. In terms of future upgradeability, for the ultimate fit, you can have custom in-ear sleeves made up for these. I got my sleeves off ACS in the UK and they made a big difference in the sound quality and isolation.
If you would consider buying used, you're getting good bang for your buck with these.
I couldn't really recommend anything else because once I got used to these, less expensive monitors just wouldn't cut it for me. That said, I did have a pair of Shures a few years ago (the E2C which are now defunct) which were a fairly good step up from ear buds and the likes - though the Triple Fi 10s are a lot better. If you want to buy new, the likes of Shure 215s may be worth a look.
Personally, I think these are the best bang for the buck that can be found:
If you like a mid-centric sound signature then yes the BA200 is the way to go but since you say you want a neutral signature the R-50 would be a great choice. Don't let the looks fool you they're a top-tier IEM where it counts and that's in sound quality.