The thing is, from economic point of view, Putin makes mistake to pay so much money just for them to participate in the union. And what Putin is known for is careful, very calculated political moves, certainly not charity. So he expects something more from it later.
Besides, saying that Ukraine will never be EU member just exposes your clear bias. Everyone knows it won't join EU for a long time, but if they elect people that work towards it (which is unlikely knowing how chaotic the opposition is) it would be a matter of time till it joins EU.
He expects to build a regional structure based on economics rather then politics in which Russia will play the main role. Portraying it as the second coming of USSR is plain wrong. It is also wrong to assume that there are plans to turn this union into political one - Russia doesn't need the headache of managing Ukrainian, Kazakh or any other nation's internal affairs. To achieve that Russia is willing to spend a few bln here and there to push them in the direction of CU. Again, EU did the same thing with Eastern Europe by accepting them on a super fast track.
I don't have bias, I am trying to see things as they are without cliches. EU is not likely to survive in its current form. One of the main issues with EU is that some countries made it there for political reasons, for which EU is beginning to pay dearly and has no appetite for Ukraine whatsoever. Putin may be making same mistake with say Armenia, but again he is just a bureaucrat like his European colleagues.
I still think Putin is not telling the whole story about his CU yet, and I think EU is in quite good shape. Only some financial problems in southern countries (Portugal, Spain and Greece) and migration problems in UK and maybe to some extent Germany. So it will most likely not only survive, but thrive further.
Besides, saying that Ukraine will never be EU member just exposes your clear bias. Everyone knows it won't join EU for a long time, but if they elect people that work towards it (which is unlikely knowing how chaotic the opposition is) it would be a matter of time till it joins EU.