I didn't have any luck doing a search on this topic: I own only base game monsters and there have been games in which I run out of monster tokens and the monster cup is empty. Usually when the terror track is at 10 and The Terrible Experiment has ended in failure. During these times, I simply don't add any more monsters to the board during a monster surge.
I witnessed a particularly odd game yesterday that my friends were playing. They had an empty monster cup and were trying to to kill all the monsters on the board. My one friend ended up with 20 monster trophies. They were refusing to trade them in for things because they didn't want them to go into the cup. I pointed out to them that I considered them cheaters because they were exploiting a weakness in the game (not enough monsters included in base game. Sorry but its true.).
Was I correct in this? Is this cheating? what should happen when the monsters run out?
Btw, my friends still got murdered by Hastur despite their antics.