Adjusted the price of plasma and rocket ammo.

There are two adjustments here:

First, Classic difficulty now gives these items the same price as
the original (50), which is important because the original made it
very easy to grind by filling up on plasma at interceptions and then
selling it. $25 is a lot more than $5.

Second, for every other difficulty, the price has been dropped down
from 10 to 1. The reason I'm doing this is unlike in the original,
you now pretty much have to routinely fill up on rockets and/or
plasma ammo to use these weapons, and it puts the weapons which
don't need ammo (laser and charger) at an unfair advantage, basically
turning use of rockets into a mistake. By making the prices very low,
you still want to conserve plasma and rockets, but the effect is more
to deprive you of their use later on in the mission than to drain
your money.

src/shop.cpp

@@ -345,12 +345,20 @@ void initShop()
 		"Increases plasma firing rate");
 	shopItems[SHOP_PLASMA_MAX_RATE].image = 11;
 
-	shopItems[SHOP_PLASMA_AMMO].price = 10;
+	if (game.difficulty == DIFFICULTY_ORIGINAL)
+		shopItems[SHOP_PLASMA_AMMO].price = 50;
+	else
+		shopItems[SHOP_PLASMA_AMMO].price = 1;
+
 	strcpy(shopItems[SHOP_PLASMA_AMMO].name, "10 Plasma cells");
 	strcpy(shopItems[SHOP_PLASMA_AMMO].description, "Plasma ammunition");
 	shopItems[SHOP_PLASMA_AMMO].image = 12;
 
-	shopItems[SHOP_ROCKET_AMMO].price = 10;
+	if (game.difficulty == DIFFICULTY_ORIGINAL)
+		shopItems[SHOP_ROCKET_AMMO].price = 50;
+	else
+		shopItems[SHOP_ROCKET_AMMO].price = 1;
+
 	strcpy(shopItems[SHOP_ROCKET_AMMO].name, "Rocket Ammo");
 	strcpy(shopItems[SHOP_ROCKET_AMMO].description,
 		"High velocity dumb fire rocket");