A number of factors.

There's earmarked 1.3 or 1.4 Billion in infrastructure changes. So thats just the roads, the mess that will be created on DLSD, and some phantom parking even further south.

Then there's demo of the last stadium renovation and most of the site, preservation of the hisotirc colonnade, and building of the multipurpose museum cmapus connection on top of the old site.

Then then,Lucas Oil is a trussed solid roof. This is a transparent ETFE (probably) roof with a far more advanced structural system. There's a lot of noise regarding the artificial playing surfaces across the NFL. I wouldn't build (and would certianly hope the Bears have considered) a stadium that couldn't long-term support a natural grass field. I imagine the previous bullet plays a siginifcant roll in the roofing choice here.

So...quickly in today's money, you rack up a big price tag. What a few billion dollars amongst friends?