I have my hotels already booked in KL and Langkawi. What is the approximate cost for food, travel and general shopping that I should budget for? (medium budget trip)
Budgeted hotel can be found in Petaling Street areas. Cost: RM80 - RM150 per night per person
Kuala Lumpur is best traveled with LRT, Monorails and KTM using Touch n Go card. Reload at the counter or at convenient stores such as 7-11 and any petrol stations
Food is just so cheap. Food from Hawker Centre could costs RM5 - 15 per person. Food from restaurants costs RM10 - 30 per person.
Kuala Lumpur is a shopping heaven and easily accessible by monorails/LRT. Few spots that you should go are Berjaya Times Square, Pavilion, 1 Utama and Mid Valley Megamall. Berjaya Times Sqr has a lot of affordable clothes (but cannot test) and its nearby areas also have a lot of international brands such as H&M.
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I actually found that KL was pretty inexpensive, probably because there's so much street food and so many cheap restaurants. Where you might get into trouble is the shopping because there are malls everywhere!
This is a difficult questions since we all have different views on what is and is not expensive. It was all cheap for me in January this year, but I as well live in one of the most expensive parts of the world back home so almost anywhere I go it feels cheap.
If you really want to get an idea about how much you pay for things, I highly recommend checking out Numbeo website. It's a database of prices for various items and services all around the world (updated frequently and quite current) and will give you better idea about cost of living in Kuala Lumpur than any subjective opinion of random traveler you might get here. That's where I go first if I want to know about prices anywhere in the world.