Does an increase in monster Toughness also mean an increase in monster trophy value?
You always apply whatever toughness modifiers are in effect when you exchange the trophy, the trophy's worth when you gained it is irrelevant.

To refer to your examples, a Fire Vampire killed before a Heat Wave, and exchanged during a Heat Wave is worth +1 toughness, while a Fire Vampire killed during a Heat Wave and exchanged after the Heat Wave has ended is worth only the monster's base toughness.

From the Arkham Horror FAQ (page 3):


  Q: When spending monster trophies, do you count any 
  modifiers to toughness currently in effect?
  
  A: Yes, use the modified toughness. This bonus applies to 
  the use of any monster trophies spent at any location.