I think Ruger still does not sell new firing pins, but outside sources do, and it just might be a worn spring problem if the cleaning doesn't cure the problem.
I have been shooting Mini-14's/Ranch Rifles for 25+ years. (Not a newbie to the platform.)
There is no firing pin spring. The firing pin channel inside of the bolt may have a lot of crud built-up impeding it's movement. Very few owners bother to disassemble the bolt and clean the internals.
If a tiny bit of the tip has broken-off, it can cause light strikes on the primers. That would require a new firing pin. Ruger does not sell firing pins, but they have excellent customer service. If you send the rifle in to Ruger, they will replace the firing pin if that's the issue. Or they will discern the problem and repair as needed. Just be sure to tell them the type of ammo you are using and that the issue is intermittent. Do not
buy an aftermarket firing pin. I have gone that route in the past, and the quality of aftermarket pins is terrible. I had an aftermarket pin break after 60 rounds. Plus, the pins need to be fitted to your bolt, which is a job best done by Ruger, or a gunsmith who is competent on the Mini-14 platform.
If you do send the rifle to Ruger, I recommend requesting a spare firing pin to be fitted to the rifle. This uses costs about $30, though the last time I sent a Mini-14 in for repair work and requested a spare pin, they didn't charge me anything.
Are they US made ammo like Remington, Winchester, etc, that is giving you the problem, or are they something like Wolf or some other kind of eastern europe/ russian steel ammo. Some companies use harder primers.
Steel cased ammo is not a problem in a Mini-14. I have put 10's of thousands of rounds or Wolf downrange with several Mini-14's with no issues whatsoever.
As far as the value of the rifle, that will depend on where you are located. In my area, a pre-580 series would have a value of between $400 and $500. A 580 or higher with a tapered barrel would be worth $600 to $700. This would be for a rifle that is functioning properly.
Magazines (especially aftermarket) don't add much to the value of the rifle. Nor do additional stocks.Gunbroker
is a good site to monitor for values. Though the prices on Gunbroker tend to be a bit higher than the prices locally.