Somebody with a background in large-scale construction can probably add more to this, but my understanding is that it costs about $1m per 10 ft. of digging and grading to start a one-block sized building. In other words, four sub-level floors equals $4m.
To remove the track, it would require 20 feet of digging and grading (about 2 floors), but the sheer scale of digging that much would probably double and maybe even triple the cost. And that's assuming you don't hit a bunch of bedrock that needs to be blasted out. Final cost just for eliminating the track--- somewhere between $4 and 6m.
That probably represents 2x what we want to spend on the new grandstand, and that is before we'd actually do any building. As much as every football fan would like to remove the track, I seriously doubt that is a legitimate option.