Not quite sure on the figures you're coming up with. We usually get over 40k, and closer to 45k, for the average group game, with sell outs in the bigger ties.
The BM game last season, which was rearranged at a days notice, had close to 35k in attendance from 44k tickets sold. Even that is around or above your predicted figures for this season. My ticket for Napoli was also £25 and not £40 as stated by yourself.
The 'Champions League not attracting our fans' doesn't quite ring true either. The truth is that a lot of City fans have never bothered with mid week cup games, for any competition, which means our crowds are significantly below what we get in the league. In fact the only mid week (non CL) games I recall selling out in the last 25 years were the 2009/10 League Cup quarter final vs Arsenal and the semi against United a few weeks later. League Cup semi's against Liverpool, Everton and West Ham didn't even sell out.
The issue of completely filling the ground isn't CL related. It's about getting fans with season tickets to go to mid week games that aren't included within the season ticket price.