A note on union leaders owning million dollar homes: http://www.ambito.com/793963-los-duenos-del-paro-y-de-grande...
This is the "most successfull" union leader of argentina phyisically removing the president of the largest private post office: http://www.perfil.com/politica/moyano-fue-a-oca-y-echo-por-l...
Turns out he owns a post office with thousands of employees. He owns a lot of things, being president of a soccer club and all. He was never anything else but a trucker and a trucker union leader.
A note on noteworthy union leaders salaries, ranging 10-15x the earnings of the workers they represent:
http://www.laizquierdadiario.com/A-vos-que-te-cuesta-llegar-...
Union leaders are not forced to publicly disclosed their tax filings, so you can only gauge what they own by what they do.
You can argue this is anecdotal evidence, but then again, theres no way to make it no anecdotal because its protected information.