for power and speaker cable all you are paying for is flexibility. ie: softer cables that are easier to work with. cause yes, copper is copper. but 4gauge power is like $10 a metre even for jaycar stuff. you'd struggle to use less than 5m in any install, 1m amp neg, 1m batt neg, 3m +ve amp - so there is $50 right there before you even start. Half the cost if you go for 8ga, which is what most people need really.
you then need 4 ring terminals ($3 ea for crimps) and two spade (ditto), a fuse holder ($15 round, $25 blade) and fuse ($5 round, $10 blade) $40-50 here.
now add 15m of flexi speaker cable for splits, and a few metres of some thick stuff for the sub - $40 easy, even for cheap stuff
then, RCA's - now this is where quality can make a difference, not silly $200 RCA's but it is worth spending at least $20 on a decent pair of shielded RCA's, jaycar 2m ones are $18 - you'll need 2 pair in even a modest install.
thats $150 for a single amp, 5 speaker setup (mids, tweeters and one sub)
it adds up - even for cheap stuff!