Tax and YQ calculation?
 

When I tried to book an one-way flight CAI-BKK on MS,
Expedia shows EUR 85,22 as YQ and Taxes.

But when I tried to book a multicarrier one-way ticket IST-CAI-BKK-ICN(with a combination of MS and OZ), it shows EUR 67,53 as YQ and Taxes.

How come IST-CAI-BKK-ICN is lower than CAI-BKK?

I would guess there is an Egyptian departure tax (which wouldn't apply to connecting flights)

http://www.highbeam.com/doc/1G1-157656113.html (Although from 2006, adds up to US$15)

if you use ITA and enter your flight details, you can get details of how the ticket price is made up - how much is local taxes, YQ surcharge etc.
(use LON for "sales city" if you want in priced in Sterling)

Some airlines have different YQ linked with different fares. E.g. LOT has YQ of 60 USD on some of its one-way fares, when with normal RT or even an open jaw ticket that does not inlove other LOT flights would only be 25 USD per segment. Rather inconvenient for me, as I wanted to book 2 one-ways.