Practical information about Wat Phra That Doi Suthep
Entrance to the temple: 30 baht
Price of the cable car to go up if you don't take the stairs: 50 baht
Note that it is also interesting to visit Wat Phra That Doi Suthep at sunrise to attend and/or participate in the offerings to the monks and have the temple almost to yourself and you will then have to reserve your red car the day before.
To go visit Wat Phra That Doi Suthep, you can also book one of the tours above.