It depends on the painters carpentry skills. Most painters will bid for basic painting only, including filling the cracks with caulk. If they have the skill, then include an option for the replacement of the siding.
This way you can decide if you want to spend the extra money now or go for a basic paint job. Ask your painter to evaluate the siding for possible replacement and make sure to walk your home with the painter. This way he/she can ask questions about how far you would like to go with replacement and offer possible alternatives that could lower the price.
An alternative is replacing only the worst pieces of clapboard and patching the rest. This could be a good option for you.