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Does it worth running a full node to a $500 to $1000 worth of smartphone, considering the constant communication over the internet, power consumption, risk to damage the hardware?
it would probably be easier to set up a full node on a computer and then connect to that using your phone, that way you are still using the full node but not running it on your phone.
as for the data usage,... you can reduce them too. for example you can reduce the size of the memory pool you have or even reduce the number of transactions other nodes send you by sending them a FeeFilter message or just disable relay to not receive anything, you can also not listen for incoming connections so you don't have to upload anything.
the result would be downloading blocks every ~10 minutes. and that will be about 2 kb/s traffic usage. and verifying about 2000-3000 transactions isn't going to be that CPU intensive either.