Publisher ripping clients off

I was approached to advertise in a business magazine with eleven years history in South Africa. I was quoted R30 000 for a half page advert + DPS editorial. Not a bad deal. I did a bit of research and when I started phoning people I knew personally that had adverts in the magazine I was blown away. Outside back cover + DPS editorial for R3000-00. (THREE THOUSAND RAND) I will never advertise in a magazine that gives others adverts of that price simply because the company has a huge brand recognition that gives out credibility. I am very tempted to phone every advertiser to let them in on this little secret. Can I get into trouble for it?