I build and sell groups and have sold well over 50.
There are some things to consider when determining what you can charge.
1) Engagement. Without engagement the group is like buying a brick.
2) Niche. If your niche is something obscure, it will not attract many buyers so you do want it to be a little broad or one that people are interested in on a fairly large scale.
3) Growth. If the group is stagnant, or growing very slowly, there is little upside for the buyer.
4) Monetization. If you are monetizing it, then you have something there. I rent the banner space, the pinned post and also slip in my own offers in my groups (I only have 14 or so now) and I also send out a greeting to them with a thanks for joining, please be sure to check out one of our sponsors, and link to a monetization method of my own.
It is hard to give you a dollar amount as to their value because you have all those factors to consider and without detailed information, which you should not share in a public forum, so offering you a number would be without any real basis.
