50 foot cables?

Mogami Sound Runner.

That is a very long distance, if you are asking about speaker cables.
Especially so when wanting something good for under $500.

Given the extreme distance, I recommend Audioquest cables with the DBS. Unfortunately, I doubt you will score a pair used at your desired length, but I think the DBS will be something that will aid in signal degradation- something that will occur at such extreme lengths.
I have tried Transparent and found them lacking, and also super expensive, compared to AQ.

Bob

$500.00 for 50ft will be cheap garbage.Make it much shorter or suffer a big sound loss.Good luck!!

Thanks for all of this. I could potentially drill a hole under our floor, run them under floorboards and instead they'd be 25 feet. Worth that instead?

Yes, much better. I suggest DH Labs T-14 in your price range.

https://silversonic.com/products/bulk-wire-and-cable/bulk-speaker-cable/t-14-speaker-cable/