It's extremely unlikely that a single press release will be a significant driver of traffic to web pages. I send out weekly press releases and get anywhere from 100 to 300 page views from 1,000 to 1,500 backlinks. This suggests that many aggregation sites have written rules that say something like : "publish to the site any press release from PR Web that contains the word _______ in the _________."
PR Web has a very high domain authority and established relationships with Yahoo News and other sites. To me, that's the real value -- along with carefully chosen anchor text in a well written press release. In the last few weeks, I've had online pick ups from The Wall Street Journal, The Times of India, and several other first tier outlets. But most are from marginal sites.
Still, the weekly press releases are paying off. I can see the results on SERPs. But this is a long term strategy that depends on spending several hours each week crafting the press release and distribution list. Expecting a single press release to cause a anything other than a small bump in page views of a landing page is almost certainly hoping for way too much....especially if it's little beyond a routine product launch.
Here's the real problem. Almost all press releases suck. They contain nothing remotely newsworthy and will just annoy the recipients, in the unlikely event human eyes ever see it. So make sure your press release doesn't suck. Or don't send it.
Also, pick up the phone and talk to a human a PR Web and ask if there are bulk discounts available.
As for the low end services, I don't think much of them. Distribution is often little beyond spamming the info@ inbox. At the other other end of spectrum is Marketwire, which often distributes directly to opted-in reporters. (PR Web offers something similar at additional cost.)
I've also gotten huge traffic from Reddit, Stumbleupon and guest blog posts. The guest blog posts delivered lower volume but higher quality traffic.
So tactics other than a press release may be a better use of your time and money, especially if you have to hire a copywriter on top of paying the PR Web fee.