Yeah I don't want to go into what I paid for my setup either. Mind you I've done a lot of upgrading over the years and wasted a lot of money as well.
If we're talking like a monthly cost I pay about $30 a month in electricity alone. Then for test kits, salt etc it all depends on how often you do the testing, waterchanges etc. I'd say it costs me about $15 a month in test kits/salt/carbon/phosphate remover.