The Trade Route UI should show you the full value of a trade route with 1 key exception (which I hope we can add one day). You get a growth bonus if your trade route goes to a civ that you have some cultural influence over. For example, if your influence is exotic to them, you get 5%, and it increases 5% for every level above that.
Another note, buildings like the caravan and customs house increase the gold you get from TRs (which is normally reflected in the UI), but this only applies to new trade routes. If you have a trade route active and build the building, that route's gold is not changed.
Internal vs External Trade Routes. Early game I find ITRs very useful. The food bonus is quite strong at this point to get your initial cities going. One thing about Trade Route bonus is that normally they are not multiplied by any of the cities' multipliers. So if your city has a 50% bonus to production for example, production get from Trade Routes doesn't get that multiplier. The one exception to this is Food and Growth bonuses, growth does magnify your food TR bonus. So one technique is timing your trade routes to go together as one. Trigger a WLTKD in your capital, set it maximum growth, and then give it a lot of ITRs with food. It will ramp up to a high population in no time.
Later in the game, its usually worth having 1 ITR that stretches through your Civ, as a trade route that goes over your roads will improve any village and town that is on the route. Only 1 TR is necessary for this bonus, any additional TR will not help further.
Once the early game is gone, I find ETRs are generally preferable, however, this is very difficulty dependent. The trick is ETRs get stronger the further behind the opponent you are, so generally the more difficult the AI you face the greater these bonuses get. On Emperor the ETRs are pretty strong, but ITRs still have a place. Talking to diety players, usually the ETR bonuses are so great that they are always using them. Its not the gold you get but the science and culture bonuses that make them so useful.
Lastly, Trade Routes to allied CS can actually be very good, often stronger than a ETR to another civ late in the game.