As a more detailed analysis of how a series of forts around Virginia could have materially aided the ability of the CSA to risk incursion, let's consider the viable supply routes for an attack on Richmond.
Functionally, there are four ways an army of sufficient size to be threatening can be supplied in the Virginia area by the Union.
1) Using the Orange and Alexandria railway and the Virginia Central railway.
2) Using the Richmond, Fredericksburg and Potomac railway.
3) Using the York river and the Richmond and York railway.
4) Using the James river.
Assuming for the purposes of this analysis that the Confederacy has control of Virginia south of the Rappahanock and the Rapidan (but not north of the Rapidan) then the following forts could be placed:
1)
A fort on the Rapidan at the town of Rapidan itself.
A fort south of the town of Rapidan at Orange Court House.
These two between them mean that opening up supply down the "western route" via the Orange and Alexandria railway involve two separate sieges or two separate escalade assaults.
2)
A fort at Fredericksburg, with heavy guns placed in range of the rail bridge area.
A fort at Port Royal which commands the anchorage.
These two between them mean that an attempt to directly cross at Fredericksburg is much harder than it was historically, and more importantly they mean that an attempt to march a large force via Germanna Ford east through the Wilderness to regain supply via the Rappahanock or via Fredericksburg (as the force would have to conduct a siege). It takes a lot of wagons to achieve a flying-column movement - Grant in 1864 used very roughly four wagons per thousand men PFD with which he crossed the Rapidan, and carried supply for less than three weeks - and every extra day of delay that a fortified position can cause is more problems for anyone trying to cross in the middle Rapidan area.
And a fort at Hanover Junction.
To use either the Central Railroad or the RF&P RR for supply means taking Hanover Junction; this fort would have to be taken to use either route.
3)
The historical fort at Fort Monroe, under Confederate control.
The historical forts at Yorktown and Gloucester Point, though enhanced with better guns pointing upriver.
An enhanced version of the historical fort at Williamsburg.
A fort at West Point.
A fort at White House Landing.
These between them would mean that it was impossible to ascend the York without clearing them out, and that it was difficult to land troops in the lower Rappahanock and cut across to close off Confederate supply to Yorktown etc (as the Pamunkey would remain open for supply). It would also make it much harder to conduct a Peninsular campaign as Fort Monroe would be a strong point
preventing landing, and without the option to land troops at West Point (at the confluence of the Matapony and the Pamunkey) then Williamsburg is a strong position at a narrow point of the peninsula that can't be easily flanked.
4)
The fort at Fort Monroe as above.
A fort at Sewell's Point, and other fortifications surrounding Norfok.
A fort at Suffolk.
A fort at Drury's Bluff.
And a fort at City Point.
The first two mean that you can't outflank the Yorktown position up the James or by a direct attack on Norfolk, and the third stymies attempts to cut off Norfolk by the "back door". The other two mean that supply up the James isn't viable even if the outer defences are gone.
With all these in place, the only viable way to attack Richmond without at least one siege is basically for the Union to have a navy powerful enough to overcome the combination of the James River Squadron and the guns of Fort Monroe and Sewell's Point, and then blast away the forts at City Point and Drury's Bluff. Since the latter in particular requires powerful, well-armoured, shallow-draft vessels, it's a tricky naval conundrum.
Is this cheap? No! But the CSA is more manpower-poor than it is funds-poor, at least while cotton is still a cash crop.
ED: estimated cost of these forts in manpower is:
1)
Two forts that can be covered by ~1,000 men PFD between them, so two regiments' worth total.
2)
Two heftier forts, so say three regiments for the pair.
3)
This needs more garrison strength, call it twelve regiments all told. (1 each for West Point and WHL, 1 for Gloucester Point, 3 for Yorktown, 1 for Williamsburg and 5 for Fort Monroe.)
4)
Say 6-8 regiments for the Norfolk area, 1 for Suffolk and 1 regiment between the City Point and Drury's Bluff forts.
Total cost in regiments:27. This may sound costly, but it's actually less than what the CSA had posted at Yorktown and Norfolk alone on
30 September 1861 (where they had 16 infantry regiments at Norfolk and 16 inf regiments plus two battalions at Yorktown).
Thus, in manpower terms, this is eminently doable.
If you don't have Fort Monroe you need more regiments at Yorktown to make sure it's not possible to "bounce" the line of the Warwick river.
Bringing the