The USGS type information IS NOT good enough for that, FWIW. The TopoDepot stuff would be suspect at best for that purpose, as well.
I'd suggest getting a surveyor. If you explain the situation, and the site is generally open, it shouldn't cost very much. If that's out of the question, you'll have to get someone with a level and do a quick grid-type survey to get elevations worth using to get volume calculations. If you need a more detailed description, let me know.
In any event, BE CAREFUL with how you get your number for earthwork. There are MANY issues involved here that can DRASTICALLY effect the number, not the least of which is the accuracy of your 'existing ground' information.
Is it safe to assume that your ground is relatively flat? You mentioned a building, parking area, road, etc. This is the type of work I do, and frankly, from what you're describing, you could be asking for alot of trouble, dabbling in earthwork calculations.