Each resource have a base price, which is simply based on the resource, some are simply more expensive than others. There is also a transportation cost, distance matter as well if the resource travel on land or over sea, it is cheaper to transport over the seas, but sometimes a land route may be cheaper due to short distance. The trade route seems to try to get to the city you have closest to the other player, so if all your cities are far away from the resource you want to purchase you will have to pay alot in transportation cost.

I think you always get only 10 money per sold resource, no matter how much they pay for the resource, however active trade routes also increase money income in cities which can be quite valuable source of income.

One thing I don't understand are the routes themselves - do you happen to know why a lot of them tend to wrap around the world vs. showing a more direct route? There has been many posts about this and I see it in all my games as well, wondering if it's a bug or there's some logic to it

Example: https://www.games2gether.com/amplitude-studios/humankind/forums/215-bug-reports/threads/44070-pc-steam-v-1-0-02-0116-trade-route-goes-around-world

Trade routes afaik map towards the fastest way to your capitol, sea or land. They may go to another city instead of your capitol but that's because once at that city it's going on a road.

Transportation costs are vastly dependent on distance.

AI is not cheating at base line difficulties and trade is harder to afford early game, nothing under the hood here about this until you are playing up difficulty. 100 to 200 gold is basically the cheapest you will see as when you compare that to trying to trade with someone across the map for over 1k a resource you are going to need really income to warrant that... Or a need for stability / strategic resource rather badly.

Lastly if you or an AI chose a mercantile culture they get more options to expedite working a luxury or strategic resource at the cost of influence and they can get a few other perks unrelated to your original question. You'll find the AI may even build the luxury for you if they want it bad enough, if they are mercantile.