My Thai husband did the same short term monk initiation in 1998. He tells me the total cost was 20,000 baht at the most. When I told him about the 300,000 baht, he laughed and said, "Are they inviting the whole village?"
So no, inflation would not explain this. It seems to me that if you really think this relationship has long term potential, you need to have a talk with your boyfriend about honesty in a relationship, because he is clearly not telling you a true story, or the whole story. If it does not have long term potential, time to toss this fish back into the river,