Not only that, they're becoming rapidly affordable. In many areas, law enforcement will carry them. They're even becoming affordable for home use ($1,400, although I believe you still need to buy through medical providers, not Costco. You might also pay $100 every year or two for replacing expired pads, and $200 every 2-3 years for a battery).
I had the impression that AEDs and CPR were for different indications. You use an AED if they're fibrillating, and CPR if their heart has stopped entirely. (Though I saw you said above that CPR might help even if they are fibrillating, just to get some blood perfusion.)
Go get trained on modern CPR. The current training is "call for help, get an AED from the environment if you can, the call center might tell you where to get it from, do CPR until the AED arrives, and then follow the AEDs spoken instructions."
If the AED detects it cannot be useful for shocking, it will at least tell you and give you a rhythm to keep doing CPR. So as a casual first aid provider, it doesn't matter to you what the AED can and can't do. Get it, get it hooked up, and let it help you.
AEDs are designed to treat two shockable rhythms: ventricular fibrillation (VF) and ventricular tachycardia (VT). (As a side note, you can have a pulse with VT.)
CPR is designed to provide effective circulation, whether the heart has stopped entirely (asystole), or is not providing effective circulation. You can even have in-betweens, like PEA (pulseless electrical activity) where the heart is providing the electrical signals that should contract the cardiac muscle, but it's just ... not happening, or not happening strongly enough to provide cardiac output.
CPR is really: the heart is not doing what it should, or it's not doing enough of it, or not doing it effectively enough.
In the case of an entirely stopped heart, CPR can provide enough sustenance to get a shockable rhythm that can then be defibrillated.
AEDs are also different from what EMS will use (at the ALS/paramedic level)—we can defibrillate or shock rhythms other than VF/VT, but the rules are different, as are things like pacing, etc.
As a VERY rough guide, a lot of arrests are a devolution, from NSR (normal sinus), i.e. all good, which can become VT, and some time in VT (typically minutes) before degenerating into VF, which is even less effective at cardiac output, and then subsequently into asystole or PEA.
