I usualy get atleast 1 a task so id estimate about 1/200, there pretty common drops but if your thinking of camping them.... tehn good luck there probably the most boring thing on rs to camp lol, i'd say hit nechryls atleast there you can just go chaos tunnels stick guthans on attack 4-5 and semi afk it, drops aren't that bad either, rune boots like 150k or something atm torstol seeds really high, good crim charms etc.