The best prices locally are about $1/watt for 24s and $2/watt for 12s in panels. But` PWM vs MPPT controllers is not such a big deal anymore as it was for PWMs that have adjustable voltages.
The big factor is still roof real estate for your particular rig. Some have big wide open spaces, and others have things sticking up all over so open spaces are smaller. You need big open spaces for 24s.
You can beat that if you cover some of those things that stick up with panels that are mounted on higher "legs", but it is all "situational" for what is possible for each case.
