I'm shopping for a new cassette. I have Shimano CS-HG50 12-32 now. I'm looking at the SRAM 8-speed cassettes: the PG-820, PG-830, and PG-850 and the Shimano CS-HG-30, 40, 50,. 70...

What's the difference? I don't care about differences in weight. I most care about how long they last: I'll pay for longer wear.

Has someone other recommendations? I'm looking in the $20-$40 price range.

I have a self-made touring bike with 24-36-52 chainrings. I'm more likely in the lower gears, going uphill, than going over 20 mph.

The weight and finish distinguish the cost levels of cassettes. If weight and finish are not a concern, any Shimano or SRAM in your budget will work fine. I prefer SRAM cassettes and use them on my road and mountain rigs.

Longevity of the cassette is a function of the chain. As your chain stretches or elongates due to use (mainly wear), the cassette teeth will be worn to match this lengthening chain. If you change your chain before it's too stretched, you will extend cassette life. See: https://www.sheldonbrown.com/chains.html (especially towards the bottom of the page).

I assume you should also replace your chain while you are at it.

You can use the SB gear calculator to figure out what kind of cassette you might want to go with. I use an 11-28 and am happy with it, but your mileage may vary.
https://www.sheldonbrown.com/gears/

As for the type of cassette and what you get for each step up, it really all boils down to the type of finish, how the cogs are stamped, how they're held together, weight, etc. Sometimes the cogs are coated in chrome plating rather than nickel or silver to prevent corrosion. For your money I think you'll be happy with an HG70, which can be found for 35 bucks at most bike shops, but then again your preference may be different from mine (I'm a Shimano guy, myself).

Hope this helps.

For a touring bicycle, you probably want to have a nice big granny gear or two in back, so I'd suggest an 11-32 or 11-34 rear cassette. As MudPie said, weight and finish are what makes expensive cassettes. It has little to do with durability. If anything, the heavier, clunkier ones may even last longer. That said, I like to use SRAM cassettes and chains on my bikes. I recommend the PG-830.

Thanks. Why the 70? It's 3 times the 30, half the price of an 80 or an Ultegra

+1 to mudpie and willjl. I use Sram 9spd, and find that the lower priced ones wear longer for me. I think they are probably steel, as opposed to aluminum or Titanium, so while heavier, offer longer service. As also stated above, chain maintenance will affect drivetrain longevity.

I think for such an integral part of the bike splurging a bit isn't so bad, plus $35 bucks isn't too bad to wing or save up for. The main reason why I went with the 70 is because the jump to the 90 is so much larger in terms of price without a whole lot of benefit to be had. If nothing else I'd go with the 50 since it's the middle of the road and a nice jump from the 30, but I think you'll be happy with anything that isn't Suntour.

The life of the cassette is NOT a function of the chain. Cheap steel cogs will most likely be softer and not last as long. Really expensive Ti cogs will wear out even faster than cheap steel.

Of course you can leave a chain in use for too long and wear even the best steel cogs so they won't mate with a new chain. The way to avoid that is chain rotation. Buy 2-4 chains and alternate frequently enough that you never encounter new-chain skip.

I don't care about price. I care about value. I don't splurge on anything. I've owned about 10 freewheels or cassettes over the last 30 years: Suntours, Shimanos, Nashbars, ? , and been 'happy' (how happy can a freewheel make me?) with all of them. I broke 2 teeth on a cog of a freewheel once, but, otherwise, I have left a chain on too long and found out only when I installed a new chain that I had worn the freewheel/cassette down.

As Jim Hightower puts it, 'The only thing in the middle of the road is a yellow line and dead skunks.'

If a 70 won't last longer or shift more easily (I can't remember having much trouble shifting in the rear that I couldn't fix by adjusting the derailleur) then it's not worth more; if the Ultegra will last more than twice as long as the 70, or more than 6 times the 30 - then it's the best deal.

Once again, paying more for a cassette is not going to make it last longer. Paying more will make it lighter, and fancier looking. Furthermore, the steel in low end cassettes and high end cassettes is the same. The fancier ones are just drilled and machined to be lighter.