The monster was always going to be invisible, nothing to do with budget according to confidential.
The monster was a metaphor for Vincent's depression. Only he could see it, and at first it was hard for people to decipher what it actually was. It was in itself blind, inconsistent, confused. It was only defeated by Vincent's easel, representing his love of painting.
I loved the episode, one of the best of the revived series since 2005. But then I'm an artist - the scene at the end summed up everything about why artists do art... you can say it's because of self fulfillment but every artist wants to think that in 100 years people will still be enjoying their work.