http://cgi.ebay.com/Kicker-KS20-Car-Stereo-Component-Tweeters-3-4-190Watts_W0QQitemZ140072426668QQihZ004QQcategoryZ14943QQrdZ1QQcmdZViewItem
that is a perfect match for your system, and within your price range.
There are several reasons. 1. build quality. 2. name, 3. reliability, 4. materials.
Sure, you have your rare exceptions, but think of it this way. If Interfire can supposedly produce a great product that handles 175RMS(which is subject to interpretation) why can't Kicker? Or Infinity? There is a reason why these companies will charge more for less RMS. Basically because it is not all about the RMS ratings. What about sensitivity, or response? How is the Crossover built? Can it really handle true 175 RMS? I would highly doubt. Try running you amp thought these speakers for a bit, see if they can handle constant 150 RMS beating (although as stated before, it really isn't 150 RMS, it is dynamic, with the supply being able to handle a constant 150 RMS signal) and see if it can handling it for an hour.
The point is there is a reason people will pay money for less RMS. Reliability, build quality, sound, and many others are the reason. I dislike my current speakers because they don't seem to handle their RMS the way I like. They distort too early, and I'd imagine the same would happen from your proposed set.