$200 million for the parking garage is to high. We know the ones at Disney Springs are only costing around $50 million each, I also don't think the re-routing of the entrance would be any where near $400 million.
It depends on a few factors, but I tend to agree. But when it comes to stuff like that, I think it's a safer bet to over-estimate. I
believe the ones at Disney Springs are 2,500 spaces each (total 5,000). The one at DHS might be 7,500 (I doubt it will be 10,000 like someone else said, but who knows). That would put the cost at around $150M, but you give some wiggle room in case they decide to go nuts and make it 10,000 spaces ($200M). The parking structure at Disneyland ran around $220M, I believe, and has 10,000 spaces.
As for the roads and stuff, I don't know how extensive the plans are. As
@Katie G pointed out above they are indeed doing some bus-only work (but I think that's just to the south of the park where the current "main" car entrance to the parking lot is), but again, I'm not sure anyone knows how extensive this will be. I do find $400M to be a fairly ridiculous figure just to reroute some roads, but you never know what might be included in this whole plan.
So anyway, my long winded point is that I agree, $600M for all that stuff is a tad pessimistic. But being conservative, if the $2.8B "total" is accurate, then $2.2 would probably be the minimum you'd expect to go towards the park itself.
Anyway, all we were really trying to do is to figure out some possible math (of how an insider could be correct about a $2.2B figure, when even though we basically know it's $2.8 in total)