Based on service of Air China you have not great opportunity to visit Singapore for very attractive fare. Open-jaw tickets to Singapore from Europe are now available already for €339 or £252. To book the cheapest flights available you have to look for flights departing in Spain and returning back to Germany (Frankfurt, Munich), Paris or London. For additional costs you may also consider to depart to Asia from Stockholm or Paris. 
For additinal fee you may also consider to book stop-over in Beijing and take the advantage of free 72h visa on arrival. If interested please look for limited travel options between November 2015 and June 2016. Blackout period around Christmas holidays. Max stay 3 months.
While the cheapest flights to Singapore are available when departing from Barcelona there is additional layover in Vienna. (in some cases). We would therefore recommend to fly from Madrid (by adding just few Euros) and fly to Singapore with one layover in Beijing.
Booking samples
The fastest way to check connection flight between above cities and your hometown is through Skyscanner. As there are many interesting routes available this is indeed an atrractive solution for most of travelers within Europe looking for reasonable solution to this destination.
Booking sample of cheap open-jaw flights to Singapore from Europe for €339 here. In this case you would be departing from Barcelona to Singapore on 15th of November. Your return flight from Asia back to Frankfurt would take place on 26th of November 2015.
