Roads - They cost you in your own territory but also if it's in no-ones territory and you built it (or controlled it last, I forget for razed cities). So CS's is foreign territory (even if not allied) so will not cost you. Incas should spam roads on hills outside territory not because it is free by default, but because their UAmakes it free.
Trade Posts. They're generally considered strong even in proper territory because
1) The extra science from rationalism
2) For tiles with no gold, golden ages will give +1 gold on that tile. So it's +1 gold, +1 gold with economics and +1 gold for golden age, wheras farm would be +1 food, +1 food with fertiliser and nothing else for golden age. (As civil service is much earlier than fertiliser or economics. and river tiles already have 1 gold for GA boost, farms are better there).
3) The building modifiers for gold (and science) are much stronger than the modifiers for food (in general, though it's concievable that you could reverse this if you planned your game specifically). Gold production is also less affected by times of unhapiness.
Planting GP is quite interesting - I think most people try to plant them on bonus resources but it's kinda complicated, based on what you could lose (the opportunity cost) from planting them at a certain location, and whether you would be working that tile anyway even without the GP on top. A few general rules are - don't plant GE's on hills, don't plant people on hills when taking order. Cows are considered the best in general. Try to plant them on a tile with at least 1 gold already there. Whether e.g. sheep are better than say deer depends on the situation more. Building on river tiles depends how long there is until fertiliser, as you'd give up a lot of food in civil service that you couldn't make up elsewhere. I think bananas are generally considered rubbish for planting great people. I think MadDjinn went over this on a beyond the monument episode so he'll probably have looked into it more than me
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edit: Oh yeah, don't plant GP on lux's or strategics, they won't be improved
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