@Aeris
I do not see how Sybil attacks relate to my question. The adversary will simply set up new nodes, without messing with attacking identities of existing ones.
As to the rest of it, let us calculate. Assuming that the adversary wants to control 4000 nodes for 3 years, the 70d startup period is irrelevant and negligible. Assuming further that operating the relays will cost the adversary $20/month each, the total "investment" required would be 20x12x3x4000=less than $3million
That’s $1million a year to control most of the Tor nodes., You call this "costly"? This amount is a joke, a trifle, petty cash for any US or Russian government agency. FIFTY times this amount is STILL petty cash, so in case you think $20/month is not enough to run a relay, make it $1000 a month.
So I repeat - how is this prevented?