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I wanted to ask anyone here if you've had a gold wash applied on a silver Sousa bell, and if so, how much would it cost?

Maybe try calling Anderson's?

Hank, it is no longer a thing because it is expensive and it rubs off very easily. A few polishings and you have bare silver peeking through the gold. As advised in the post above this one, Anderson Silver Plating is a good place to hire out.

If you decide to pull the trigger and have it done, take "before" and "after" photos and post them so we can take a look at the difference. You could also post a write-up telling us what Anderson had to do to prepare bell and then apply the wash.

Best of luck!

It has become quite expensive to do this.
When the silver itself has been badly worn down, I’ve often seen silver bells - which of had the silver buffed off of the interior - and then the brass lacquered, for a faux gold appearance.
The source mentioned in previous posts is the correct one, if you decide to follow through with genuine gold.

Yamaha special gold lacquer is really close to gold wash. That's what Harv did on my old 46K Grand Jumbo sousaphone

Nikolas has a gold tint that can be added to their clear (which would be mixed and sprayed out of a gun), as well as rattle cans with gold tint in them.
I do not recommend rattle cans for a surface as large as the inside of a sousaphone bell, but knock yourself out.