As always in China, try and avoid public holidays and weekend (specially for the Leshan giant buddha). Book a car transfer from Chendu city to Leshan in advance through your hotel (you will need a very early start)
Plan to be at Leshan Giant Buddha ticket office at opening time (8 a.m.), which means leaving the city of Chengdu at about 6 a.m. You will beat the crowds and the heat, and be back in the city in time for lunch.
Avoid high summer, which is very hot and humid.
The most expensive item is the private car transfer from your Chengdu hotel to visit the Leshan Giant Buddha. You can also reach Leshan through metro, then train, then taxi.