So as @dirvin says - a node has to go through 1024 churn events if it is to have age 10 and beyond - and if we are to stick with his assumption of a churn event per 30 minutes:
We get 48 churns per day, and 1008 churns in 21 days - so if a malicious actor connects nodes to the system 24/7 for 3 weeks - those nodes will be at age 10, which frankly is nice age number to pass the quorum requirement of being above than 50% of age of the group (because when a node goes offline they lose half the age, so these malicious nodes would be at a nice point in age and rank after three weeks of 24/7 connection).
Hence a node cannot cost more than $10. I agree with you, the more expensive every node is - the better it is for the security of the system. But I think ageing and getting a high rank can be achieved in 3 or so weeks.