how much should you spend on cable?

It's a little difficult to answer that question because I stopped using cables as well as interconnects and power cords a while back. But good luck with this thread anyway. 😀

Here for some cable science: http://www.roger-russell.com/wire/wire.htm#introduction

There's that science claim again. 😩

A person with limited funds should buy the most expensive cable they can afford. 

There's more truth than humor in that statement. Think about it. 🤔

All the best,
Nonoise

From the Roger Russell screed on everything you never wanted to know about audio cables:

KIND OF WIRE MADE NO DIFFERENCE

It can be solid, stranded, copper, oxygen free copper, silver, etc.--or even "magic" wire--as long as the resistance is kept to be less than 5% of the speaker impedance. There is no listening difference as long as the wire is of adequate size. Bear in mind, as previously mentioned, a well-designed amplifier will not have a problem with any of these wires.

That's terrific. Now I can stop worrying so much about the coat hangers I'm using for cables.

You should spend between $10 and $20. If you spend more, then you are wasting money.