Sara owns a paint shop that makes custom paints. To make one gallon of a specialty color, she needs 4 royal blue units that each cost $b. She also needs 9 yellow units that each cost $y. A customer orders 4 gallons of this paint. Sara uses the expression 4(4b + 9y) to find the total cost for the color units needed to make each gallon of paint.
Explain how to write an expression that is equivalent to the total cost expression. Then determine the total cost for the color units needed to make the paint if each royal blue color unit costs $0.30 and each yellow unit costs $0.10.