I love Z cars.
Does her car look like mine? If so it is a 280Z and not at 280ZX.
240Zs and especially ones with early build dates, are worth considerable more than 260Zs and 280Zs. These are also the ones Nissan bought back and restored in the late 90s. They spent $40k per car on the restoration and sold them for about $25k as new cars w/ warrantees.
My Opinion:
-A driver 78 280Z with ok mechanicals and body is worth $1500-2000
-Good Condition/Well maintained=$2000-4000
-Restored Garage Queen/Low mi orig owner=$4000-8000
If it is a 280ZX, take those prices and divide by 2.
zcar.com has some good info too.