this is very difficult to say unless one knows the condition of the rifle, the caliber, with or without the magazine, and if there are sights on the rifle. Being "99% new", probably in a popular caliber of 223, 243, 22-250 or 308 I gustimate that the rifle is worth $275 - $450 depending upon the buyers need for the rifle
1000$
$350.00
100-400 USD
$500-$600
100-400 USD
the 243 was new in 1969 its 375.00 in 100%
100-300 US Dollars
I will first say that the Remington model 788 was made in 6mm from 1969-1979,at this time they are going for anywhere from 250-450 dollars depending on condition.
Just paid $350 for 788 in 30-30 caliber.
Remington did not make a carbine in .44 Magnum caliber. They did chamber the Model 788 rifle in that caliber. Do you have a Ruger carbine in .44 Remington Magnum, or perhaps a Marlin in that caliber? sales@countrygunsmith.net
100-400 USD
Consider yourself lucky if you get 300 dollars for it.