Sweet spot for PSU efficiency generally is operating them between 40% and 60% of their rated capability. You can look this up, see corsair for example.
The 80+ precious metal efficiency rating...
The basic 80 Plus rating means that the PSU is rated for at least 80% efficiency at 20% load, 50% load, and 100% load.
The 80 Plus Bronze rating means that the PSU is rated for at least 82% efficiency at 20% load, 85% at 50% load, and 82% at 100% load.
The 80 Plus Silver rating means that the PSU is rated for at least 85% efficiency at 20% load, 88% at 50% load, and 85% at 100% load.
The 80 Plus Gold rating meas that the PSU is rated for at least 87% efficiency at 20% load, 90% at 50% load, and 87% at 100% load.
The 80 Plus Platinum(bet you didn't know there was a Platinum) rating means that the PSU is rated for at least 90% efficiency at 20% load, 92% at 50% load, and 89% at 100% load.
the higher efficiency PSU's cost more money, the depressing feeling resulting spending the extra money on one is supposed to be offset knowing that when you leave your computer on it's wasting less power.
What is the cheapest possible PSU I can get without the risk of destroying my RX 570 GPU or any other components in my computer?
I think you found it with that seasonic 500w for $43. Seasonic is good, 500w is plenty. I wouldn't hesitate to get a 400w psu to run a 120w tdp rx570 and 95w intel i5. You're not likely to find any new good name brand reliable psu's for much less than $50. A good psu that is new will cost at least $50, it's just the way it is.
Can I use an old (10 years old), generic, working PSU (my current one) that is like $10-$15 cheap that has 600w, but combine it with a surge protector and not risk my components from being fried?
a surge protector that the PSU plugs into (then surge protector plugs into wall) will not prevent a PSU going bad internally and possibly breaking your computer. Your existing 10 year old 600w psu would likely be fine. you do not require a new PSU especially if it's in an existing pc and your just adding an RX570. You can buy a new psu which can go bad, don't fret it. If you're existing 600w psu has been working up till now there's no reason to think it won't continue to work. Post the make/model of your existing 600w psu.
A full modular psu is nice because you won't have a rats nest of psu cables in your tower, if you care about such a thing. It can make working inside a tower, especially a small one, much easier. More of a visual appeal thing. There is nothing more reliable about a modular cable psu vs non-modular one.
a 250w tdp RTX2080ti running farcry5 pulls around 350w on a watt meter; max power draw observed was less than 500w (~460 if I remember) running full out stress test to see what the max power draw possible was. Your 120w tdp Radeon RX570... any psu over 500w is wastefull and I would not overlook 350w or 400w psu's. But you're not likely to find them much below $50 anyway.
There's a rash of counterfeit cpu's and graphics cards having old chip's firmware rewritten to be resold as a newer model. Careful who you buy from if using amazon or ebay, which I would not. Stick with a known retailer/reseller.