Thank you, gentlemen and ladies.
But I have to say, it's a good thing I was ignorant as to the value of the vase/jardiniere, or else I would have ended up paying more... The sellers wanted 150zl for it which I thought was terribly overpriced. When I handed the vase back, one of the sellers told me it's a 1930s art-deco piece. I started laughing and told him it's 1970s. After a quick exchange in Russian (not with me, but with the other seller), his partner told me that I may be right, and that he'll give it to me for 100zl. I told him that that's still too expensive and started to walk away. He asked me how much would I give him, so I yelled out 'not more than 60zl'. He said he'll sell for 80zl, and I said I'll give him 70zl. He agreed.

It's only when I got home and saw these two links that I realized the vase/jardiniere is worth more than I ever imagined:
http://www.liveauctioneers.com/item/4269088http://www.auctionatrium.com/index.php?page=view_item&itemID=6724Even 100 GBP is much more than I expected.

I really do feel as if I won the lottery.

px -- the vase is 14cm high and 14,5cm wide. It weighs about 1,4kg. It really is an impressive piece.
I am one very, very happy woman.