Without a doubt, spend the $150 on a custom molded pair....
I have to respectfully disagree with rokpunk here, I have a set of custom molded plugs that I just got a few months ago, and I was fitted by the president of the largest manufacturer of custom molds in the country. She even made the plugs herself, so they are done correctly and fit great
but, for years I've been using the Ety ER20 style plugs:
or you can get the 15 dollar ones http://www.etymotic.com/ephp/er20.aspx which also work quite well.
and I can't discern that much of a difference. In fact, the ER20s actually attenuate a little more than my custom plugs with 25db filters due to the fact that I'm not shy about jamming them pretty tightly in my ears.
In my case, because I'm doing some prototype testing for the company I got my plugs from I got them for $50, and it was definitely $50 well spent, if for no other reason than to be able to give them a head to head comparison.
The custom plugs do have a noticeably better frequency response, but unless you have the dough to toss around, I suggest staying with the $15 ER20s.
My $.02
Shane