In my experience, PR companies help with messaging more than press releases. They help your company to have a cohesive message so when you talk to press, it is very clear what your comapny is doing. This can be very helpful as it can be difficult to explain technical concepts to a general audience. They give media training to prepare executives to talk to the press. They also draft speaking and awards submissions for your company.
Pricing varies and sometimes they charge retainers and other times they charge hourly fees. It can be a big expenditure for a startup, but PR companies often have relationships with reporters that can help get your name out there.