a Xantrex Prosine 1800 ? does that not cost more than $6-700? As far as I remember it does cost that much because its a 'true' sine wave inverter. Couldnt you just get an Opus ?
If you get a pseudo sine wave inverter (hardly more than 50-70 bucks), you will definitely get noise through the AUX input of your headunit (assuming thats the setup you have in mind) so take that into account.
I installed my inverter on the back of the rear seats. Because my rear seats can fold down, I have access to the outlets even from the interior of the car.
but the fan on my 'cheap' inverter makes a lot of noise which I can even hear when the rear seats are up.
just my 2c.