I just finished my todays contrabant raid..226 000 for Opressor lvl 17 (17 is it? Not sure) and if I can give you advices, here you go:
1.) They are not worth it! If you want dekas just to show off, go for it. They are like sports cars: fast, unpractical, cost way too much. Droidekas hit hard..but they break on stronger wind, repair time is ridiculously long and they cost way too much. Even my level 27 Sentinel dies in every defense!
2.) You cant keep too much! Dont try to have big suplies of contrabant. I dont care about that pink drug unless I can spend it right that time. So I usually start raiding for contra just few hours before my droid is ready.
3.) Dont stop! Once I start raiding for contra, I dont log off until it is spent for next Droideka upgrade. A time management is important here.
4.) Choose targets (and planet). Nobody have days for farming. You have to chose planet where many good targets are and skip bases with low contra. If you sre attacking everything that comes to you, you wont get above lvl 20 of Dekas.
5.) You cant defend it. Doesnt matter how great your layout is, or how high your turrents are, they will get it! Honestly I think that too strong looking layout is bad here. Average players will skip you cuz they will see no chance getting it. That results in being attacked by pros that will clear you like nothing
Those are my experience, but I ain't lvl 10 and my Droidekas have long way to become lvl L
so I might be wrong.