This indeed is a not so easy question - meaning that to directly have a correlation between press release submission and an increase in business is not fully possible.
However, a press release, well taken out in the media (issued, and consequently covered in the mainline and vernacular media) does have a mileage in terms of increased visibility among target audience - be it a product or service!
Just to illustrate, if your product is an off the shelf FMCG product (say a soap or detergent), once there is media visibility on that - could be launch, or could be that you are issuing a release with the latest rejig or relaunch details, there is bound to be a spur in demand.
In our own experience at The PR workshop
www.prhq.in we have seen this happen many a times. On some instances when we had taken out vernacular press releases, mentioning a product free sampling event across some schools (for a womens cosmetic), the distributors in the geography reported a spurt in demand for the product. This was corroborated not by just the client, but our own independent research too.
Besides spurt in demand, there for sure is a significant WOM (word of mouth) on the product or service, in the immediate aftermath of the media information exercise.
The caveats in this - your release must be well timed, and well written. And the consequent media mentions must be a few at-least, across mainline and vernacular media.
Also, the PR measurement of the outcome per se, will be a challenge, and may not be accurately quantifiable.