Yet another way to consider the OP's question, which has already been well-discussed. This is just something I heard from a training video that I think makes sense here.
Addressing RKBA, not setting it aside, no training required. Government-required training to exercise a right is wrong for the same reason poll taxes and literacy tests were wrong.
Being a responsible gun owner however, you should at least get formal training, even if it's only a short 1/2 day class, in gun safety and the fundamentals of operating your firearm.
If you intend to rely on your firearm as a means of protection, then in addition to knowing gun safety and how to shoot, you need to know how to fight with a gun (or with the guns you'd use), and, when to fight (the legal stuff).
Armed citizens with little or no training are constantly saving themselves from attackers despite not having "enough" or even any training. (David Grossman in The Bulletproof Mind presentation- recommended). Your chances of prevailing though are much better if you've had at least some training. Your chances should be better yet if you've invested in more and higher-level training. There are no guarantees though. You could be trained to the highest level there is, and still be hurt by an attacker. But if you can do it, you owe it to yourself and to those who might rely on you for protection to get trained. That sounds sappy but it's true. Take it from Lt Col Grossman which is where I heard that particular truism.
So, if you can't afford any training, then you can't and you might be ok. I'd submit though that investing (paying for) at least some training ought to be in the budget though right behind a gun and ammo for it. I didn't always live this, I used to be one of those people who thought he knew what was going on and didn't need to "waste money" on training. Fortunately I wised up eventually and got on the path of training. (I'll be on it for the rest of my life) I don't take groceries off my family's table to pay for training, but I have taken luxuries out of the budget before to pay for it.
I'd argue a good minimum level of training for an armed citizen to be that corresponding to going through NRA Basic Pistol, (and maybe Shotgun and Rifle, too), NRA Personal Protection Inside the Home, and NRA Personal Protection Outside the Home. That's more training than the vast majority of gun owners will ever get. But, that amounts to the BASICS of what you should know. I teach all these NRA classes and think they're great, but, if you have the means, I'd say to go further yet and take the basics of shooting and personal protection from one or more high-level instructors, whoever they might be. I mean places like Thunder Ranch, Gunsite, Rangemaster, Massad Ayoob Group, and many others like that. (No disrespect intended to any establishments not on this partial list of examples).