What is the value of a 303 British?

Assuming you mean a rifle in caliber .303 British, and not the ammo- depends on which rifle, and its condition. A SMLE (#1 Mk 3) or a #4 Mk 1 may be worth $100 for a poor condition rifle, or $350-400 for an Excellent one. The TRUE Jungle carbines have their own values (many counterfeit copies) and other models different values. Rifles that have been "sporterized"- converted from standard military configuration are worth much less.