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For cheapest price paid in each market
Q1 2.38 iron for each dollar paid
Q2 3.57 iron for each dollar paid
Q3 3.57 iron for each dollar paid
Q4 3.57 iron for each dollar paid
Q5 3.40 Iron for each dollar paid

How these figures are calculated.

I go to each market level look at the cheapest product that has over 50 pieces of iron selling in it.
Then I select the maximum I could buy from that company lets just say it was 90 for each.
Q1 90 Iron .42 USD per/iron
Q2 90 Iron .56 USD per/iron
Q3 90 Iron .84 USD per/iron
Q4 90 Iron 1.12 USD per/iron
Q5 90 Iron 1.47 USD per/iron

So then you take the amount of iron pieces you are buying * the price this is just one important figure
Q1 90 Iron = 37.8
Q2 90 Iron = 50.4
Q3 90 Iron = 75.6
Q4 90 Iron = 100.8
Q5 90 Iron = 132.3
Now you must remember that number after the Q actually means this Iron bought*Quality
so for Q1= 1*90=90, Q2=2*90=180

Q1 90 Iron = 37.8
Q2 180 Iron = 50.4
Q3 270 Iron = 75.6
Q4 360 Iron = 100.8
Q5 450 Iron = 132.3

Now lets make every dollar amount= $1 by dividing both sides by there totals
90/37.8=37.8/37.8 ) =2.38 Iron per $1

Q1 2.38 iron for each dollar paid
Q2 3.57 iron for each dollar paid
Q3 3.57 iron for each dollar paid
Q4 3.57 iron for each dollar paid
Q5 3.40 Iron for each dollar paid

I will update these amounts daily if I can weekly is a promise, as you can see buying in the Q2 to Q4 range will net you the same amount of iron per dollar you spend on it, my suggestion go for Q4 if you can.