Yaron - the PR agencies you cite in your question are the giants of the industry, and if you are (as I suspect) operating on a tight budget they may not be the best choice - Shaike who has answered your question gives a good estimate of what would be a reasonable fee for a small company operating on a tight budget, but it may be too low for the bigger players to feel they can work within.

While you are not specific, your product sounds interesting - something simple and easy to use with practical parenting application that fulfils a key need could be very compelling.  I would recommend that you consider first and foremost whether the PR players you approach demonstrate clear ability to use PR strategically to both deliver against your initial launch objectives and to help develop and grow your business.  Great PR firms begin by developing the right and most effective PR strategy for you, which will then determine which media and exposure choices they make to execute the PR plan against that strategy - eg, should your product be carefully 'seeded' first to build a critical mass of loyal users who can be made to feel that they have 'discovered' it themselves before it is then taken out to mass market exposure, and to ensure there is a stronger story when pitching for such highly competitive exposure opportunities as Martha and Oprah? 

It's hard to recommend which PR players you should approach without knowing much about your product, but you might also wish to look at analogous products or launches whose ability to generate PR coverage and buzz you have admired, and find out who was their PR resource or agency.

 Very best of luck with it.