Where there's muck there's brass
Where there are dirty jobs to be done there is money to be made.
Brass has been used as the name of copper and bronze coins, and later of all forms of money, in the UK since at least the 16th century. Joseph Hall's Virgidemiarum, 1597 includes:
"Shame that the muses should be bought and sold For every peasant's brass."
'Where there's muck there's brass' is a 20th century expression which originated in Yorkshire, England where brass is still used as a slang term for money. The expression is rarely used nowadays, although writers sometimes call on it when they want to establish a character as a blunt Yorkshireman. By 'muck' any form of dirt or manure may be implied, depending on context.
John Ray expressed the notion in A collection of English proverbs, 1678:
"Muck and money go together."
The expression was preceded by the 'where there's muck there's money' variant, which dates from the mid 19th century.