This is the old and famous "bait and switch" fraud. You were induced by an advertisement (the email invitation) to $50 deal. You got email clarification--if I understand what you wrote--that the assignment paid $50. That's a contract.
Legally, you, as the independent contractor accepting as assignment, are a "consumer." You get all the consumer protection, such as it is, in your jurisdiction for accepting a business opportunity. See the Code of Federal Regulations, Title 16, sec. 238. [http://www.ecfr.gov/cgi-bin/text-idx?SID=59cfb9cbb9ff1d719627a6b202059812&mc=true&tpl=/ecfrbrowse/Title16/16cfr238_main_02.tpl]
I wouldn't waste your time with the FTC. They'll brush you off. Your "damages" are $10. But if you can find just one other victim in, say, Florida, Little RICO will empower you to sue for treble damages or $200, whichever is greater plus "reasonable attorney's fees." The false advertising "bait" went out by email, which is wire fraud, and they are paying you through the mail or PayPal which is mail fraud. These are "predicate acts" of a Racketeer Influenced Corrupt Organization.
If it happened to you, it has happened to others. You still sound reluctant to make a stink, and it's probably not worth it for $200. But if you ask an attorney like Gerard Gibbs, [http://www.girardgibbs.com/deceptive-advertising/], they might be quite interested in the Class Action implications of your complaint. Class Actions earn a fortune for the attorneys; not so much for the plaintiffs; and are extremely painful for the defendants.
It's up to you of course, but a basic rule of democracy, Constitutionalism, and the law is that you have NO rights whatsoever unless you are willing to stand up for them and have them vindicated BY the law.
I feel that I was baited & switched last month by a scheduler who sent an email ostensibly to me alone, with my name in the message, showing no other addressees or copies, begging for help on a list of 14 jobs. That came Thursday, Christmas eve at 5:12 PM. I replied that I would be happy to do five of them in Chattanooga (120 miles away); two in Johnson City (95 miles away in the opposite direction) on the next day; one up the road (10 miles away). Four of those jobs were advertised at $30 with a $30 bonus. That reply went to her at 5:37 PM EST (she's likely in California) on Thursday Christmas Eve.. Her reply came at 11 AM Monday saying "You are scheduled."
When I opened the roster, she had scheduled me only for three $30 jobs in Chattanooga. None of the $60 jobs. That got my Irish up and I told her that I reject those three jobs, spending 4 hours on the road, plus the time for the rather detailed shops, all for $90 when the IRS business mileage rate alone is $142! That to me is bait & switch, an unfair and deceptive business practice. I intend to pursue it and have the offending scheduler "corrected."
Good luck and your mileage may vary.
OldFox