If all you want is to turn on your PC on, with no regard for the state of your Auto (or the carputer), you can easily do so with a $2 switch or a $5 relay.
Realistically, however, getting a startup/shutdown controller is probably much more affordable in the long run. Our price also includes a Bosch Relay and wiring kit.
If your time is valuable, using a pre-built solution will save you from numerous headache's. We've been in this industry for 5 years now, and we're certain that engineering your own solution will cost you much more money in time and effort.
Regardless of your outcome, we wish you the best of luck on your endeavor.