BioMed Central and PLoS charge between US$ 1,350 to US$ 2,250 to publish a
paper.
Other open access alternatives should be in the same range.
Higher than that should ring alarm bells that something is wrong.

The cons that involve travel to speak at a conference remind me of the
mid-1980s first pass at what are now called "Ted Talks".  I was asked to
help fund the event by a mathematician named Benoit Mandelbrot.  He was a
buddy, he was going to speak at the conference, and really wanted me to
pitch in on "something new", so I did personally, and got the company to
contribute too.  He had yet to become semi-famous in the obscure way geeks
become "famous" (for making "fractal" a term (mis)used by nearly everyone
for nearly everything, until the term became essentially meaningless).  The
conference was real, but everyone involved ended up spending far more money
than expected, including the speakers.  The organizers were not very
organized...  Then we got angry letters from the sound guy about not being
paid, and claiming that because we were "founders", we were responsible for
the debts of the organizers of the conference.  Just because we were thanked
by the organizers.  Uh huh.

On the other end of the spectrum, more than one person buttonholed me when I
stepped into the breach to try and teach some basics to the flood of people
who decided that they would get a hive of bees, and thereby help to save the
bees, bees being the first "endangered species" one could mail-order.  The
question was always along the lines of "What's the catch here?  Why are you
teaching this class for free?  Are you selling bees?  Hives?"  I had to
explain no, I wasn't selling anything, and that I was teaching in defense of
my own bees, an explanation that made no sense to the questioner, who did
not (yet) understand that he himself was the potential threat to my bees. 

And then there's the most expensive bee talk ever given, where I merely
drove from VA to PA to talk to an assemblage of beekeepers, and had the
bottom end drop out of the engine of a 1966 Jag XKE when I was driving home.
Nobody's fault, but the honorarium did not come close to even the shipping
fees on the parts I had to buy.

So, not every invitation to pay money is a con, even when it looks more like
a con the closer you get.
And even when you are not asked to pay money, you may end up doing so.

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