The value in a Millionth Vega is that it is the more desirable early 71-73 Vega.
I recall about a year or two ago there were several stock, original/restored early Vega.Vega GTs for sale - we tracked them here in the forum on h-body.org. One went for about $8,000 and others in the $4,000-$6,000 range. I think.
So, a low-mileage or restored Millionth Vega in extremely good condition is worth not much more than these prices. That's the absolute high-end considering the economy today. The low end is as low as free or even negative territory - you get paid to take it away.
As I have said many times, rare does not necessarily mean desirable - in the overall automotive marketplace. You will never see one of these on one of those auction TV shows. Nor as an 'extreme makeover' candidate on TV or in a magazine.
But, are these cool cars? Absolutely.
Most people just don't know what we know here on the h-body forum.
Good luck on your purchase. Give a fair price to the seller, post the pics here and become a regular on this web site. You won't regret it.