Recently, I've had a few friends who have approached me about sketchy sounding publishing deals they were being offered.
This is not a new problem. For example, for years, there have been people who advertise that they are compiling a book of poetry. They request submissions, but in order to be included, you have to pay. Then, you must pay additional money to receive each copy you might want to own or distribute to friends.
However, with the advent of self-publishing, the problem has escalated. It can be very hard to tell the difference between a self-publisher that's genuine (like CreateSpace or Lulu or many if not all the major Jewish ones) and someone like Publish America who doesn't charge you up front, but then demands extraordinarily high prices for any "needed" services (like editing) and then overcharges on shipping every copy you want and sets prices so high that buyers are turned off. Not only that, but the wrong publisher (even some traditional ones that are big names) can sneak things into the contract that limit your rights regarding overseas sales, translations, ebook or film rights, and so on.
Additionally, even many legit self-publishers will forget to tell you that the average self-published book will never sell more than 100 copies--so it is highly likely you'll never recoup your investment unless you study book marketing and prepare to premote yourself like crazy.
If you want to be taken seriously as an author, there's another concern: there's less prestige associated with self-publishing than going the traditional route. (By the way, this is less true in the Jewish world, where it has always been common to self-publish, co-publish or to pay for publication through donations.) However, self-publishing isn't the no-no it used to be so long as you are sufficiently informed about the process.
The lesson from all is this is do your research BEFORE submitting to a publisher or agent, and certainly before signing any paperwork. These two sites are must-sees for anyone contemplating a book or article submission:
Preditors and Editors