Hi Friends..
I just wanna learn to play piano. Is it difficult to learn and how much it does cost? I would like to hear you guys opinions.

Hello,
How difficult any instrument is to play depends on a lot of factors. Do you have any musical background at all? How much time do you have for learning and practice? Do you want to teach yourself or go through a teacher? Are you dedicated to your goal? etc etc.
I am currently teaching my self to play piano. I am 58 years old but have been playing music since I was 12. I am using the Piano For All lessons (I think the download was only about $45) that are a good starting point and very cheap. You can google it for availability. You can also find a lot of resources on Youtube. Bill Hinton (check him out on Youtube)also has a good basic book for about $15 that teaches you how to improvise more than just learning how to read music.
I am finding that it takes a little while to get both hands working together, mainly because it takes the brain a while to build the connections you need to do it.
I say go for it, have fun and don't take it all too seriously.
I bought a Yamaha YPG235 76 key keyboard for about $250 that gives a good piano sound. Something like that can get you into it at a reasonable cost just in case it doesn't work out and you then have a piano to deal with.
If you love music give it a try.The main thing with any instrument is practice, practice, practice. You will get there eventually.

Thank you for your useful advice. These questions I must ask to myself before going to start learning.

I started learning how to play the piano at the age of 17, and believe me, it's so fun and easy. You can be 7, 11, or 70, but trust me, the piano is one of the easiest musical instruments to play and learn.