Are pay announcements worth their price to hire people?
Yes, it's worth paying for an advertisement if you end up hiring a stronger candidate.  I tend to be fairly frugal, but I think of your situation in terms of one-time cost vs. recurring cost.  When you pay for the ad, you pay just once.  When you hire a weaker candidate, you pay every day in terms of lower productivity and lower quality results.

In the U.S., a typical annual salary for a postdoc in math might be in the range of $40,000-45,000 (33,000-36,500 €); in CS, it would likely be higher.  Even the IEEE price (the highest of those you list) is roughly 1% of that.  So, if you're fairly confident you'll get a better candidate by paying for a few ads, IMHO, you'd be foolish not to do so.