In one line
The closest thing to the raw media price you pay is CPM (Cost Per Mille, "mille" = thousand) — the cost per 1,000 impressions.
Why it matters
When CPM rises, it usually means the auction got more competitive (seasonality, competitors upping bids) or your targeting got more expensive. Since CPC, CPI, and CPA are all downstream of CPM through CTR and CVR, tracking CPM on its own helps isolate one specific cause when costs rise.
What to check before reading it
CPM varies by country, audience breadth, placement, optimisation event, and even day of week within the same platform. Comparing this week's CPM against another campaign's absolute number is far less reliable than comparing the same campaign against its own earlier period on the same settings.
Following the chain with numbers
$1,000 of spend against 500,000 impressions is a $2 CPM. Add a 1% CTR and you get 5,000 clicks at a $0.20 CPC.
Once the chain is visible, a rising CPM stops reading as automatically bad. If CPM climbs to $3 while CTR climbs to 1.5%, CPC is still $0.20 — you paid more for the placement and it worked proportionally harder. Reverse it and CPM holding at $2 while CTR falls to 0.5% doubles CPC to $0.40. Cost metrics read in isolation point the wrong way.
Go deeper
How CPM, CPC, CPI and CPA connect through the funnel is covered in CPI vs CPA vs CPM vs CPC.