I apologize if this has been exhausted elsewhere on FlyerTalk, but can someone with expertise in this area please explain some of the economics to me?

The most obvious case for code-shares seems to be to provide a coded flight for onward connections to places that an airline doesn't serve -- e.g., DL code-shares with AF for flights out of CDG, so one can book a Delta flight from IAD-CDG-XXX. This seems to especially make sense in countries in which it is not so easy to buy tickets for another airline and/or the airline doesn't offer e-tickets, e.g., traveling from RUH-FRA-IAD on LH-UA, it's difficult to buy a United ticket in Riyadh, so there's a (slight) benefit to having the whole thing coded as Lufthansa.

But there are some strange aspects. I ask because I am on a DL-coded, AF-operated trip now IAD-CDG-DXB, returning IST-CDG-IAD. When I wanted to make a change to the return, my travel agent told me that, although AF had availability IST-CDG-IAD for the day that I wanted to fly, there was no more DL-coded inventory available for this itinerary. This was Y fare basis, so I am assuming that it wasn't because I was on a cheap ticket. I have never seen this before. So, how do the economics work? Does DL essentially buy a bunch of seats from AF and then re-sell them on their own? Why does Delta code-share with many IAD-CDG-XXX AF-operated flights, but not with IAD-AMS-XXX KL-operated flights? Similarly, it seems like every domestic NW flight out of DTW has a KL flight number, but no domestic UA flights out of IAD have LH flight numbers. How is it in Air France's interest for me to buy the ticket from Delta? Reciprocity on some other routes? Getting American travelers to choose to connect in CDG (on AF) for onward trips to Europe? Or is it in fact the case that having a DL flight number increases access to these flights (and sales) for U.S.-based travelers?

Another strange circumstance, for which I do not understand the rationale for code-sharing, is that most (if not all) United and US Airways flights in the U.S. are coded both airlines, and sometimes it is significantly cheaper to buy an entirely UA-operated itinerary with entirely US-operated flight numbers, or vice-versa. (I have seen the same thing with Emirates and Sri Lankan out of Dubai.) Or, for a UA-operated flight, sometimes US flight numbers will be available but UA flights won't be. I wonder what the underlying economics of this are that make sense. And, given the price transparency available nowadays (at least for domestic flights), how could it be that the prices are so different in these types of cases? It's not like the only way to buy an airline ticket is to go down to the Airline X city ticket office and buy whatever they're selling coded on their airline. Perhaps people flying CLT-ORD are connecting either in Charlotte or Chicago, so the respective airlines hold some of their coded inventory to support these connections (e.g., the different supply and demand for XXX-CLT-ORD on US versus CLT-ORD-YYY on UA might result in different prices for people buying CLT-ORD tickets depending on whether it's US or UA coded)?

Of course it comes down to supply and demand somehow, and of course airline pricing is incredibly complicated, and I assume that airlines are operating rationally, and I understand the benefits of code-sharing in many cases, but these quirks puzzle me. Does anyone have a Codesharing 101 primer?