I'm primarily looking at pathfinder golems and 5e golems, and the differences between them and I've been wanting to make some constructs in game, but 5e is severely lacking in construct design, the real problem I'm having is understanding the how's and why's of making them.

 For instance, the iron cobra costs 1000g for the metal to make it. In some places it says it's magically treated, but the price of iron is like 2 silver for a pound if it, though I don't understand why that's required when no other magic items need to be "treated". Next, while I understand the cost if you don't make it yourself, why does one still exist if you craft it? for instance, the normal iron cobra costs 5000g to be made, however, why is the cost only halved (2500g) if you roll to make it. Are you paying the construct itself? It doesn't make sense. 

Similarly, while they cover turning golems into (nerfed) shield guardians in pathfinder, these golems still require huge fees, and in 5e the building of golems basically isn't explained at all, just a time and gold requirement. Shield
Guardians have 0 explanation entirely

Questions:
Is there anything wrong with using pathfinder golem building in 5e?

Where do the obtuse prices and time limits come from, in game or otherwise?

Any exceptions, rule advice, explanations or comparisons are welcome.