If a WW2 Mauser is in great shape, and does not have an import mark stamped on it which yours won't if your grandfather brought it back as a souvenir, the collectible value may be in the $350-$400 range. Higher for rare variants.
But the price will be determined by the year it was made, who made it, is it complete, do all the numbers match, overall condition, etc. Many variables.
Can you post some good quality pics?