You can get a more detailed breakdown of your trade income which tells what you get from sea trade, land trade etc. Its worth monitoring over several years, not just immediately after building a market in 271 when the trading world is small.

I find that early in the game infrastructure is poor generally so trade is worth a bit less, but as ports and markets open up (even the Eleutheroi seem to build them) my trade income soars. Trade agreements with everybody you aren't killing right now is a must.

It seems take ages for the effects of a newly built market to kick in. Port improvements seem to give an instant and dramatic boost as they give you an extra trade route straight up (so long as there is an available trade-agreed port), so the income can rise by 50% or 33% or whatever.

I also notice once I have a developed trade economy that war with my trading partners devastates income: this is a wonderful bit of game engineering which realistically makes war less desirable. Trade is absolutely vital for non-blitz long term play IMO.

As for the morale thing, maybe it is just a false impression but troops from places with wonder and temple morale bonuses definitely seem to have more staying power and I make them a priority for troop production.