Read up on the Ear/Headphones forums to see what people think about the various kinds of cans that are out there.
I personally like the Shure E2c earphones that I got a couple weekends ago for ~US$65. You've got quite a budget there, so you have a bit of a selection. You can get something like the E2c's and have some money left over for (a nice, expensive) dinner, or shop for something a bit closer to the end of your price range (Shure E3c's or Etymotic ER•6 Isolator earphones).
150? Ok! LOL, if you're really really serious about sound (as it seems you are), you should not look for consumer level earphones. Since you're willing to spend big time, go to a specialized store in recording stuff. Certainly they will have the serious things. Look for headphones instead of earphones for the best damn quality. Actually, the huge things are the best. You will lose all portability though, and probably you'll need an adapter because these ones come with large plugs. Again, only if you're really really serious about it (Then again, this would be serious headphones for mp3's -unless they're at 320 kbps-, which would be kind of contradictory...)
I bought Shure E2C today.
For $58 Including s/h, no tax at www.buy.com using 10% coupon.
I bought it because
it was cheap, and had good ratings from ipodlounge
and people who were using them did like those earphones.
Shure makes great products, You should get it.
If you don't care about the $$$ get the Shure E3C or E4C.
If i were u i would get a pair of Grado headphones for 120 Canadian dollars, they sound great, or get a pair of Denon headphones, these are not in ear but i thing these work great with all types of portable palyers, and they are more durable