user upgrades: factor out gold/platinum prices.

app/logical/user_upgrade.rb

@@ -0,0 +1,13 @@
+class UserUpgrade
+  def self.gold_price
+    2000
+  end
+
+  def self.platinum_price
+    4000
+  end
+
+  def self.upgrade_price
+    2000
+  end
+end