I read somewhere that he did the voice for A Grand Day Out for the princely sum of fifty of your English pounds, which must have been quite a fortune for Nick Park as a struggling art student. It's stuff like this that demonstrates that he is a true artist - yes, he could have asked for thousands, but like Nanny Ogg says,
"The price for being able to shoe anything, anything that anyone brings you, is having to shoe anything anyone brings you."
The price Sallis paid for having
that voice was...well, having that voice. You've got a gift and if you want to keep it you have to pay your dues. Some people get it, some don't. Sallis clearly did. A true gent.