Edited by bmeat - 8/11/14 at 11:08am
Heya, don't get hung up on the tech used (e.g. BA vs Dynamic), just go for what works.
IEMs are regularly rated at ~26dB isolation, but at $50 most of your options will use silicon tips. In my experience, their seal is a bit inconsistent--I recall having to fiddle with such IEMs until I got the fit just right, every time I wore them.
IEMs that use foam tips (such as Comply tips) will generally give you a better seal and thus consistent isolation, even while moving around.
An alternative is to get those ear mufflers (sorry if I'm using the wrong term) that drummers typically do, solely for isolation, and wear earbuds or IEMs underneath that.
If you need isolation, the only real good way is IEM (in-ear-monitor) with such low budget.
The majority of the forum is usually heavily focused on consumer/audiophile not special-purpose... You will have hard time finding information about drummer headphones.