Buzz marketing in FB groups

Adding 'advertising' of our store to 20 (selected by us) thematic groups on FB.

Of course, in order for an advertisement not to be immediately recognized as spam, you should:

1) join the group and lead previous conversations with people on the group

2) place the advertisement / product placement in a reasonable place on the group where the conversation on the given topic will take place

Topics - decor of children's rooms, interiors, kitchen

I am asking for a price for adding 20 ads, under assumptions 1 and 2.