Generally, multi-city air fares are much higher than round trip fares. Let's say I'm getting these flights (A->B->C->B->A) and I am staying in first B and C. It is actually a round trip with additional stay in first B. How can I get the best deal on this journey?
There is really no 'trick' to this type of travel. If you want to book this as a single itinerary then the only 'trick' is to go to as many booking sites (brokers and airlines) as you can manage and do price comparisons. It helps to have some flexibility in the dates.
If you don't mind booking multiple tickets, probably the best deal on this trip will be to buy two round trip tickets. First A->B->A and then, separately, B->C->B with the latter ticket being within the time you spend in city B.
The main issue with this is that these are independent tickets. If flight A->B is delayed, causing you to miss B->C then you are responsible for buying a new B->C ticket.
Since you plan on staying in B before proceeding to C, this will only be a problem on your return journey where a delay in C->B can cause you to miss B->A. Ideally, you should split your stay in B to ensure against this. At minimum ensure that you have 3+ hours stopover on the way back (more if the flight B->A is an expensive one).
Lastly, it is worth pricing individual flights from A->B, B->C and C->A. Typically, you wont save any money over two round trip tickets, but sometimes it may wind up only costing a little bit more and you get a simpler leg C->A. This will vary depending on what cities A, B and C are and how they relate to each other. Also if an airline has its hub in A and offers flight to both B and C (but has no flights between B and C) they may sell you a return ticket with the legs A->B and C->A.
This all depends on what cites A, B and C stand for. I did a trip like this recently where A was Reykjavik, B Washington D.C. and C San Francisco. It wound up being simplest to book A->B and C->A (via Seattle actually) as a return ticket with one airline and then B->C as a separate one-way ticket with another airline. Doing it this way was actually a bit cheaper (although only a bit) but the flight home via Seattle was much shorter then to go back home via D.C.
Another option that doesn't appear to be mentioned is to book a round-trip ticket from A->B, and a round trip ticket from B->C. The result is that you have an A->B->C->B->A flight with two return tickets, which often works out cheaper than booking one-way tickets or segments (in Europe/Australia/USA, not so much in South America, however).
If it's getting complicated, a travel agent or Flightfox (crowd-sourced expert search engine) might do it for you, as they generally have more knowledge on how to lower the prices and what's available. Of course, they'll also charge you for the service :/
Just want to share a few general tips:
Get any idea about how much it will cost to put everything on single ticket: Most travel websites supports multi city search. Just put down your itinerary and then search. Write down or print out how much it costs, and which airlines operates between cities.
Next, breaking up the itinerary into two roundtrips. A -> B -> A and B -> C -> B. If C is not a popular destination, or B -> C is dominated by a different airline alliance than A -> B's alliance, this could be much cheaper. Also, this could be a good option if B -> C has many low cost carrier options.
If you just want to stay in B once, then look for open-jaw flight for A -> B, C -> A, and separately an oneway ticket from B -> C. This could save money since you will fly less.
If B is a hub city, you can just search A -> C and see if there's any flight connecting at B. If there's one, you might be able to make B a "stopover". It's trickier to buy stopover fare online, but if you're lucky to spot one, the stopover fare could just be the same price as the roundtrip from A -> C -> A.
Use alternative airports: many people underestimate the importance of picking the right airports. If you're going to an international segment followed by some domestic trips, it's possible that all the cheap domestic flights are using another airport in the city. Don't forget to do your homework to figure out the right airport to search.
Oneways: this is less common for international flights, but in rare case the airlines just won't give you a good price if you buy roundtrip. You could just buy oneways for all segments.
Look for alternatives other than flying: if you're going to Europe or Japan, high speed rail could be another great way to travel. It's possible that taking a train is just comparable to taking a flight, and you might find the train fare more attractive.
To name a few sites that could help you figure out the above:
- Google flights: best for searching multiple city flight in one itinerary because of it's fast response time.
- Kayak: Great way to compare flight price from multiple sources.
- Vamo: specialized in multi destination trip optimization with most tricks above built in.
- Skyscanner: Great LCC coverage
- Rome2rio: Quick way to figure out transit options from one place to another
- ITA Matrix: A powerful flight search engine to find the flights and learn their fare rules.