The problem is when you build a new stadium, it can't just be seats. You need to incorporate corporate boxes for sponsors and hospitality as that helps generate money. As much as it looks ugly, having the stadium double for a athletics track or be able to host events other than football is also important to help generate money for the club. I.e Spurs new stadium is going to also be used to host NFL.
Just because a stadium cost circa $20m elsewhere, doesn't mean it'll be the same in Wellington. There are lots of hills, regulations for earthquakes have come in, different council, it needs to be somewhere with good transport links, etc.
But at the same time you don't want to be too far from the action either. We have way too many Cricket/AFL grounds that are used for soccer matches and the atmosphere is just sooo so poor. So if we have a running track, we need to make the stands real tightknit to the running track.
Take Lakeside Stadium (where South Melbourne play) for example. There's the same amount of excess grass outside of the sidelines as Hindmarsh Stadium, and then just running track.