Yeah, $300 labor is totally outrageous if doing it yourself is even a remote possibility. The DV7 isn't too difficult to get apart. I managed it pretty easily without any type of service manual, but then again, I never intended for mine to go back together. If you decide to go that route, here's what you should try:
The first thing to do is clearing CMOS. To do this, you should take it apart, removing all external power sources including the main battery. You will find a small lithium button cell either on the motherboard somewhere or connected with a short cable. Remove this and let the computer sit overnight. It isn't a bad idea to push the power button once to drain any capacitors that may still be holding a charge. Then you should put it back together and see if it worked. Chances are that it won't, but this really should be the first thing to try because it costs you nothing. If it doesn't work, you'll need to replace the motherboard.
If you decide to do it yourself, see if you can acquire a service manual. It will give fairly explicit instructions about how to work on it (or at least it should. I've never used an HP one before.) The part can be purchased from HP as iPwn said, or various websites. Make sure it's for the identical model, not a different dv7-series computer. eBay could also be a great help if you're cheap. Functional used motherboards pop up fairly frequently there, so you shouldn't really have any trouble finding one.
Alternatively, if you know anyone who is really into electronics, you could ask them to try to reprogram the BIOS chip for you. Anyone with a background in electrical engineering should know how to do this. The motherboard is trash anyway, so it's worth a shot. Make sure it's stripped down to only the motherboard though. Doing it wrong could kill your CPU or memory if they are left attached. This may or may not work. It really depends on what caused the BIOS to become corrupt in the first place.
Obviously, make sure it isn't still under warranty before you do any of this. And contact HP even if it isn't. It's not unheard of for companies to do repairs on out of warranty stuff when something is so blatantly screwed up like this.
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