Q: Is it wastage and a sin to buy a 900$ phone? Thanks

A: If one has a need for such a phone would use the phone with all its necessary features, then in such a case, such a technology would prove a blessing and one may even gain reward.

By contrast, if the $900 phone is acquired as a status symbol, to impress people, then in such a situation, one will be converting the blessings into a sin.

Allah Certainly Knows Best.