Here’s how these <$100 full-size ‘phones compare with the MS1:
AKG K240S – warmer sound, more refined (and "sweeter" midrange, more bass quantity, but bass is slightly over-done and treble isn’t quite as open
Audio-Technica ATH-A500 – tighter bass, more top-end extension, but mids sound somewhat plasticky and treble can be a tad harsh
Beyerdynamic DT231 – warmer midrange, but treble is recessed and ‘phone sounds muddy in comparison
Grado SR80 – more aggressive sound, but treble is less refined and bass is looser
Sennheiser HD25-SP – more refined midrange, tighter bass, but bass doesn’t extend and treble is muted (I'm not referring to the HS25-1, which is a much better headphone, IMO)
Sennheiser HD280 Pro – deeper bass extension, but mids are recessed and the can lacks impact (and I find it very uninspiring)
Sony MDR-V6 – more bass quantity, more forward treble, but bass is overbearing and treble is way too harsh for me
Sony MDR-SA1000 – more detail, tighter bass, larger soundstage, but a tad too harsh and a bit thin for my tastes
The above cans represent viable alternatives to the MS1 and all hover at or below $100. However, the Alessandro MS1, Grado SR80 and SA1000 are the only open headphone amongst these and even then only the SR80 and MS1 could be considered portable (and the MS1 is a better-sounding headphone than the SR80, IMO); therefore the MS1 is the best semi-portable open headphone under $100, IMO.
From the above list based on pure sound quality (no other considerations) these are my top 5:
1. MS1
2. K240S
3. SA1000
4. ATH-A500
5. HD25-SP