Difficult to answer, but I'll try.
I actually had a very good experience with a (small) industry analyst firm. Besides good conversation with them, they helped us doing a video-podcast interview and mentioning our company in quite a few opportunities. I remember, in a specific period, Google Analytics showing us a lot of visitors in our website were coming from the analyst blog.
That said, it's very difficult to estimate a ROI for these kind of investments.
If you already have customers, get in contact with them and ask them directly. How do they learn about potential new products? Do they read sector magazines? Do they go to conferences? Do they rely on suggestions coming from developers?
That should help you to understand how to connect with other potential customers.