Hi Rose, re your comments about having to "make a booking" before you can get the upgraded flight cost. I always upgrade on flights outside of Europe. Depending on the trip, maybe just premium economy or maybe business class, or maybe just for one leg, eg return from US.
How it has worked for me is that you make a provisional booking for a particular holiday, JY charge you £100 initially - but the booking is conditional on getting the required upgrade at a reasonable cost (to me). It usually takes up to a week before JY come back to you with the cost of the upgrade options you have requested (I sometimes asked for quotes for both premium and business class, one way (if a particular leg looks a bit more onerous etc). But usually if I haven't heard back after 4/5 days I might phone up to see if they have the costs/find out when I can expect them.
When the quotes come back, I then decide what I want to do - in my case I've always found that there is some option that is acceptable - and at this point I book the holiday and the full deposit is taken.
I know sometimes, for some people, the airlines have quoted ridiculous upgrade costs - and obviously if this happened on any holiday I was planning on booking I would decline (and JY would refund the £100).
I am off to Burma at the end of October, have booked business class for the return flight Heathrow to KL (ie the longest flights on the trip) - but the rest of the flights are in standard class. I'm going to Cuba in February, but for this trip only upgraded to Business class for the return trip (the overnight flight), including the outbound flight was a stupid price.
Hope this helps a bit.
Regards
Katy