I had the same problem about a year ago: $309 to repair.
A year on and the problem is back again.
Quote from Canon (Australia) this time: $190
Quote to dismantle unit, supply and fit listed parts, adjust as required, reassemble, clean, check and test.
YG2-2169-010 POWER DIAPHRAGM ASS'Y $ 22.33
YG2-2140-000 FOCUS ASSY USM, $ 96.46
Labour $ 41.00
Parts $ 118.79
Freight $ 13.00
Sub Total $ 172.79
GST $ 17.28
Quotation total $ 190.07
After a number of emails they have decided to repair it for free this time, as a sign of goodwill.
Canon 70D w/Grip l Canon 60D w/Grip l EF 100-400 F4.5-5.6L IS l EF 70-200 f4L IS l EF-S 15-85 f3.5-5.6 IS USM l EF 100mm f2.8 USM Macro l EF-S 18-55mm f/3.5-5.6 IS STM l EF 50 f1.8 II l EF-S 10-22 f3.5-4.5 USM l 430 EX II Flash l Manfrotto 055XPROB + 498RC2 Tripod l Benro MP-96 M8 Monopod l Lowepro Vertex 200 AW Backpack l Lowepro Pro Runner 300 AW Backpack l PS CS5 Extended l Lightroom 4.3