In 2019 I completed a piece of research that looked at how sensory rooms are used across the UK and identified key features of successful rooms and also barriers to successful use of the rooms. (Some were to do with how the rooms are set up, most were to do with how they are used).
The research has just been published in the Tizard Learning Disability Review Journal (if you have access).
The book Multiple Multisensory Rooms; Myth Busting the Magic (www.thesensoryprojects.co.uk/books) looks at the findings in more detail and gives a history of the rooms since their inception.
My Facebook page @TheSensoryProjects has a photo album full of improvised sensory rooms other people have created, some for less than £100.
You absolutely can make a fabulous sensory room on a budget. You can also make a completely useless one for thousands and thousands of pounds.
I would be very happy to chat more to you about it. All my contact details are on the website mentioned above. I was due to be running a training day “What is happening in our multisensory rooms?” now, but Lockdown stepped in! In the afternoon of that day we were going to build improvised sensory rooms!