I'm not greatly familiar with this topic but this is what I suggest.
- For starters: Don't Pay For Content. Not even for guest posts.
In my opinion, you should just be able to ask someone to write a guest post on a certain topic, maybe you think someone else's point of view might be better than your own or you just don't want to write about it. You both will benefit from the post if it is a well written post, it is marketed correctly and linked back to the authors site.
I think the ROI can be represented by traffic increases and user engagement. If you are running adwords then you might find a monetary return that way. If you are selling a product and the article helps direct readers to your site or product you can monitor the monetary gains that way.
Setup Goals tracking in google analytics and track visits through article to your products (you can create a funnel all the way through checkout if you want). I hope all o this makes sense. like I said above, I'm not really familiar with guest posts. Oh and don't forget to return the favor for the guest author!
I just did a search on moz for "guest blogging". Here are a few article links.
The ultimate guide to advanced guest blogging
Getting Started with guest blogging WBF
Why guest posts and blogging is a slippery slope