Some extra notes on Bede's trades.
I am, from an educational pont of view, very happy that Bede posted the no-trade update for 875 BC. We would have made a profitable trade, but waiting proved more profitable. The team got a chance to see that.
I am, from an educational point of view, very happy that Bede did an non-optimal trade for Const, Poly and CoL. My first reaction when I saw the situation was to buy Const (which Bede didn't). He gave me the opportunity to point out that my hunch was correct and gave me a way to show you how to handle 3fer1's.
Bede is a bit sloppy with money. For instance: he paid 345 gold to Spain, where 342 would have sufficed. This seems small; but in close trades it can mean a lot.
The amount of money William received for Const has nothing to do with what he wants to pay for the later techs. The increase in revenue had to do with Const being a near monopoly (after we bought it) and we could use that near-monopoly tech twice to obtain Poly and CoL + the Spanish and Greek treasury.
I never use CivAssist. I don't know the beaker cost and I don't know what each civ values. Instead I just open the F4 screen and check what each civ offers for my techs or wants to pay for mine. In fact I think the CivAssist knowledge hinders people and distracts them from the purpose of trading. Bede is an experienced player but even he "messed up" a good 3fer1.
What I do is check (with CivReplay; a tool that shows what techs what civ has clearer than the F4-screen) what techs the civs have and lack and how much money (and later on resources) they have. Then I make a plan on how I would want to trade. I check if that is possible in the F4 screen (the first step at least) and then I take the most important step: I stop.
I rethink and contemplate (and check) alternatives. If for instance in my original plan I count on being able to pay gpt or resources (or obtain them), but the prelimenary results show me otherwise it might be needed to choose an alternative.
In this game this isn't the case (yet) as lump gold is always accepted by the AI. But if our gold-amount is lower or the prices rise we might need to pay gpt or with resources.
When I've finally picked the trade I prefer then I do the trade, rechecking after every step. Civs may hold invisible techs or adjust their prices, which influence the optimal trade. Don't be afraid to change mid-trade; just make sure you know what you want in each individual step as well as what you want as final result.
To answer O-P's question: "My first instinct is always to acquire techs to lower the cost of the monopoly, here CoL and poly for Cons, but to see the huge difference going the other way means I gotta remember that. So why in this situation did it work this way. Rik said usually the other way works better and it's how I would try it but what about this situation caused the change?"
If 2 civs would have had Const at the time and we'd have to buy as the 3rd player; the value of Const to Spain and Greece would have been lowered as a consequence, thus they would probably not have offered all their techs + money for it. The value of Construction in "Rik's Alternative 2" was that this expensive tech (for Spain and Greece) was used to obtain 2 non-expensive techs (as more civs had it then Const).
There is more to it, but I can't explain it in words. You've seen the benefit of buying of a near monopoly tech over the far from monopoly techs and using those techs to obtain the near monopoly tech.
In general:
For a 2fer1: Buy the most expensive tech from a civ with cash; use it to obtain the 2nd tech and then sell the tech to the first to get some money back. (If you buy the cheapest tech, you still have to pay extra when you use it to obtain the expensive tech.)
For a 3fer1: Buy a (near) monopoly tech with cash, use it to get the other 2 techs and use those 2 techs to get some money back.
For a 4fer1: Go for the tech closest to monopoly, use it to get the next closest monopoly tech and use them to obtain the others. Then resell for money back and even profit.
A 2fer1 usually costs some money, a 3fer1 is about break-even and a 4+fer1 usually is (big) profit.
Instead of cash you can use additional maps, resources and alliances (RoP, etc) as well, if the game is like that.
Of course; and that is what makes trading fun; general rules do not always apply...
