50 foot cables?

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

@OP, 
You might get lucky searching Ebay for used cables.
25 feet is long, but I scored a set of AQ Rockefeller cables a while ago.
They were the real deal and AQ cut them down and had them reterminated when I moved to monoblocks.
Also, check Hifi Shark. Use the filters to narrow down your search.
A little patience, and some luck should allow you to find a pair of cables.
(Just keep in mind, there are counterfeits out their, so do your due diligence. -That is try to find sellers who can provide a receipt of purchase from a dealer, if possible).
Bob