The Substitution Variable Prefix can be set to the dollar sign ($), but this generates an error within TPerlRegEx "Error in regular expression at offset 6: nothing to repeat." when attempting to execute a query.
Toad for Oracle Professional (64-bit)
Add-Ons:
16.3.231.2085
Context of where you're using this would be nice (e.g. in an automation job, etc.)
However, I wonder if you have to use an escape character?
That's something we might have to do internally if it is set to a $.
I haven't had time to look yet - trying to get some things done for version 17.0.
In the meantime, June, my only advice is the obvious - pick a different character for substitutuion variable prompting.
Exactly what I've done once I realized what was happening 
I had this issue a couple of months ago and went through support (in that case we resolved it by removing the user files folder and letting toad recreate it assuming that something got corrupted), but today I was trying to avoid the substitution prompt since I'm working with data with a number of ampersands in it and happened to stumble on it again and had that 'aha' moment of realizing the dollar sign was causing a conflict. Not a big issue at all, but wanted to share in case it snags someone else along the way. Thanks!
I definitely appreciate you reporting it. I'll look at it when I get caught up and hopefully we can make it "just work".