To add to the post above, let's say you surround an 11x11 wide tower, with one block breathing room between the base and the pit, with 192 iron spikes, 3 rows across in the pit surrounding the tower. Iron spikes take 4 forged iron to craft, so that's 768 forged iron for the whole lot - that's approximately 1 1/2 stacks of forged iron. By default, it takes 12 iron (plus some clay) to craft one piece of forged iron, and unless I'm mistaken, iron that is put into the input of the forge comes out to be a 1:1 ratio. This means it will take 36864 iron to craft the required amount of forged iron to craft/upgrade those 192 iron spikes; that comes out to 6 stacks + 1,000 iron. As stated above, it takes a third of an iron spike to down a regular zed, and two thirds of an iron spike to down a regular feral. Best case scenario, let's assume that half of your spikes are used. In this scenario, that should net from between 200 - 240 zombie kills on average, assuming they're all basic and none of them make it through. However, to apply this situation realistically, let's say 10% of zeds are beefier than your standard Arlene or Darlene, so this drops the kill count to about 190 - 220, to be on the safe side. Total potential kill count will probably come out at around 200 zeds on average for 96 total spikes, if the scenario ends in a perfect streak of zed carnage. This is unrealistic however, so expect some zeds to make it through your defenses, walk around areas where spikes have already been taken out, or multiple zeds trample on a singular spike simultaneously, creating diminishing returns.
Assuming however that you miraculously, through some strange and almost perfect coincidence, get approximately 200 zombie kills for approximately 100 spikes being used, on average. Are you willing to spend 18,500 iron per 200 kills per week? Up to you.
Edit: To be realistic about it though, count down the total number of kills per week from about 200 to maybe 140 - 160 on average maximum. Again, you have to take into account zombies brushing up against spikes but not actually dying.
Note: I was doing some crude math here, plus it is 2 in the morning where I am, so take all of these numbers with a pinch of salt, plus individual circumstances will vary greatly since entropy abound in these kinds of situations and order is few and far between.