So do cables really make a big difference in sound quality? What is the best/least expensive cable to buy? Are the monster cables really worth the money?
My Gear:

Fender 62 RI Stratocaster
Epiphone Les Paul Standard+
Fender Supersonic
Boss DD-3 Delay
Boss OS-2 OD/DS
Martin 000-28
Washburn D10s
Cables make a difference, yes, but I wouldn't buy the most expensive ones. Going from the cheapest no-name cable to a medium price cable will make a much bigger difference than going from a mid-price cable to a really high-end one.

My personal favourite cable that is easily available is planet waves. Stay away from Monster, they are one of the most overpriced brands in the industry.