The Sixers recent offers to Oscar have been, in my opinion, not terrible and on a year-by-year basis, their offer was certainly not half of that offered by New Zealand. Had Adelaide put their offer to Oscar during the season (I believe his agent approached the club to do this, but not 100%) I think he may well have signed it. As Brett Maher implied on Talking Hoops last night, if you don't want to lose a player, you don't let them come out of contract (Maher said that he'd never come out of contract himself).
It is not about the money - Oscar has turned down offers to play NZ NBL in the off-season to play for lesser-paying Sturt. I don't believe that the financial situation of BSA/Sixers influenced his decision strongly either - in hours of talking about his decision he didn't raise it.
Lemanis and Dorge have a very good reputation at developing players and Oscar is someone that needs to be pushed. I hope it's a very positive move for him in that respect.
So, while there's a bit of an apparent pall hanging over the Sixers after Rychart leaving and Ingles picking the Dragons, and I think it could still be a challenge to recruit from here, it's not the end. Maybe they slipped up in letting his contract finish, but when another team has bigger plans for a player, or offers a different approach, you can't always match that.