ZAG-BKK in LX Z and return PEK-PRG in LH L for €1374 total
Another possible risk: You might lose the possibility to change/cancel the ticket if you add cheap nonflexible fare.

Cheapest B-class I found is return DEL-IST on OS for €1795 total

Or you can return PEK-BUD in LH V for €1516 and have only €100 cancellation/change penalty (Better deal than B class)