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I changed my url structure from subdomain.site.com/keyword/ to site.com/subfolder/letternumber.
Everything else has remained the same.
CPM dropped immediately and has remained constant at about 50% of what it's been the past several years.
Any suggestions, other than going back?
Could it be due to the new url not having the same number of inbound links the old one had?
The new url still has pr greyed out, which scares me. It's indexed in google. A link: search shows no inbound links.
You're not the only one who's cpc is cut in half,
Shortbus1662 is talking about CPM (presumably meaning eCPM), not CPC (advertiser's cost per click) or EPC (publisher earnings per click). A decline in eCPM can be caused by a drop in either the average EPC or clickthrough rate (or, in some cases, both).
In Shortbus1662's specific case, it would be helpful to know whether EPC, CTR, or both are down. Knowing that much might help to shorten the list of possible causes, but we'd still be stuck with guesswork, because we wouldn't know why Shortbus1662 was seeing a decline in EPC, CTR, or both.