I really don't think $150 is bad since you are just starting out and don't have an established clientel yet. Not to mention, this is someone you know personally right? Thing is you have to do some work at a very reasonable price first to establish your name and ability. Once you do that then you start to raise your prices, not to mention that the more you do the more efficient you will become and the 8 hour detail will only take you 6 hours. So even if you are still charging the same end price you are making more per hour because you are more efficient.
The same detail I do for $175 right now I started out doing for $50! I started out there and people jumped at the price, but you know what the same truck I started on for $50 the guy now pays me $175 for detailing the same truck, for the same detail process.
Now one thing I will state is that if you are going to start doing this for other people you really should look into some sort of insurance policy. I have one because you never know what might happen. If you accidently drop a buffer on the hood or God forbid burn through some paint, you are screwed. Not only will you not get paid for the detail but you are going to have to pay for a repaint. I like having the peace of mind that if something freaky happens, and it probably will if you do enough cars, I'm covered.
Oh and good idea on not doing the interior. Interiors are money pits and the first place you start to lose money when detailing. I mean it's not unreasonable to spend an hour vacuuming a car that probably has never been detailed and if you are trying to get $40 an hour, who in their right mind would really pay $40 for you to vacuum their car. You don't expect to spend as much time on an interior as most people do because it really is time consuming. If you are doing a really good job on the interior then you need to tack 2 hours of labor onto the cost of the detail. I'm straight up with my customers that I won't spend more than an hour on the interior for the price I quote. If they want more or the interior is that important to them then the price goes up.