Okay this has been bugging me for a few weeks now, so I thought I would take the ten minutes it takes to figure it out.
For this example, I am building a hypothetical 10m long fence, which is 800mm high. Now, given that we are using the same 100 by 75mm treated pine sleepers as the rails, and we need the same amount of concrete, aggregate for drainage and ag pipe etc. The only point of difference price wise are the actual posts.
Assuming that we use the standard 2.4 lengths of rails we need 5 posts set at 600mm deep.
For the steel that would be 5 steel posts at 1.4m long at
$30.27 per metre for a sub-total of $212 add another $25 for the cost of the 5 cuts and the total is $237
For the treated pine posts it would be 5 treated pine posts at 2.4m long at
$26.40 each (I have assumed the retaining wall is built for longevity using the standard 100 by 200 sleepers) for a sub-total of $132. Then we need to add the cost of the galvanised coach bolts. Once again doing it properly there are 4 per rail times 16 rails at
$1.60 each for a sub-total of $102.40. Which gives a total cost of $234.40.
Now, I'm not yet finished. There is an unfactored time cost for drilling, and bolting up the 64 separate holes, the wastage of about 5m of treated pine, and the durability of the steel over the treated pine.
Make sense?