Rick,
Whats up bro. I just dropped my car off at the Lexus dealer about an hour ago to get this very thing fixed. I have a good friend who works there so he is going to look into the price and all that and get back to me... Here is the deal:
First, lets make sure to be clear that we are talking about the 6x9 front door speakers which are located closest to your feet.
Now, Lexus knows good and well that these speakers simply crap out over time. Thats just the way they are. Weather you blast your stereo at bass +5 or not, these things just seem to break down over time. What happens is that the speaker woofer separates itself from where it is supposed to be. Technically speaking, you may have not "blown out" your speaker... Rather the wear and tear over time has caused a separation of a particular part and that is causing the raspy/blown sound that is so dam annoying (trust me, I hate that too!).
Next, the system in our cars have an amplification impedance of 8 ohms/16 ohms subwooofer. Its pretty much impossible to find a speaker identical to the ones we have installed. Surely, you can install a 6.5 inch 4 ohm speaker (which is possible but you would most likely have to fabricate your own housing for it) or another 6x9 4 ohm. If you do this, your sound will definitely not be as high a quality, you will lose bass sound quality, and possibly blow the amp.
**Don't listen to what random ass people tell you. They will try to tell you its possible and all, but the physics of it is simply not so. You cant defy science.**
Here are your 3 options (please keep in mind that this is my opinion after my personal research):
1) Check out this link. Its the best I have ever been able to find.
http/clublexus.com/forums/lexus-audio-video-security-and-electronics/288367-speaker-install-sc430.html
This dude replaced the speakers, ***but as you can see, he also replaced the amplifier and wiring as well.*** This process is quite expensive depending on the quality of product you choose in that you have to pretty much replace everything and rewire those speakers to a new amp. The end product could be nice, but make sure you know whats up cause it seems like an intense process.
2) Use the link above to access your speakers. Remove them, and re-glue the broken part. I have read that people use some kind of adhesive from Home Depot and its works well. Just research and see if you can find "how to re-glue speakers" on Google or something. Also, there are companies out there that will fix/repair your broken speakers. They cost around $25-30 each to do it, but you will be without your system for quite some time (with respect to shipping back and fourth plus the work itself).
3) Last, just bite the bullet and go to the dealer. However, $800 is way too much!! Shop around different dealers or even try to contact the factory to see if you can get those speakers. Thats just the way it is bro. We chose a high end option, and it has a high end cost to repair.
I have done hours of research on this, and I think I know my stuff. This was the best advice I could give you from what I have learned. I wish you the best of luck in getting your problem resolved.
Take care,
Kevin