Cheapest item for Ecotron?

Oh yeah, i forgot about roses.

You can manage to get a lot of thems.

Stool, Rubber ducks or mats. Brown plasto used to be good but it's becoming more popular.

Use roses they were free for some time!

But 1 cred items will work too. :)
IMO i'd prefer to trade other people for them, it's much cheaper that way.

if you have norms like roses you could try sell them for 2 other norms such as stools and get more furni that way as well.