Check to see if there is a letter ahead of the serial number.It should be their and then we can identify the year of production.
Your serial number indicates the year 1987.
Your serial number indicates that your Marlin model 782 was made in the year 1977.
You will need to add the serial number,to get the answer to your question.
Made in 1963
1976
Your serial number indicates that your model 782 rifle was made in the year 1981.
1978
Your Marlin model 783 was made in the year 1971.
150$ to 250$ is my best guess
Marlin switched from Ballard rifling to the newer micro-groove on their 81 DL model in 1954.
My Glenfield Model 25 has a micro-groove barrel,so I imagine others do as well.
$350-400 in PERFECT shape.