When someone is selling a nice car for pennies you should ask more questions, not mutter bargain.
There are probably numerous factors at play.
Giants gave him an extension, he is contracted for 2017 with a salary of around $300k (which I assume we are going to carry) and he isn't going to play in 2017. That frees up the list spot and some cap space.
Players with serious injuries that require a lot of rehabilitation also take up a lot of time and resources to manage that a fit and healthy player wouldn't.
Giants don't have a shortage of talent coming in, at present, they could realistically have asked a lot more for Ahern but they likely wouldn't have used the pick they got on anything other than academy fodder and they will likely have more than enough points once they clear out a few more players.
Ahern wanted to return to Melbourne.
Giants probably don't see our club as an immediate threat for their current side, they would probably rather trade him cheaper to us than for him to end up a club that might be in competition with them over the next few years.