What do you think would be a good budget for buying a smartphone?
Roughly how much money do you think I should spend on getting a new smartphone?
ご質問ありがとうございます。
① "What do you think would be a good budget for buying a smartphone?"=「スマホを買うのに良い予算は何だと思う?」
② "Roughly how much money do you think I should spend"=「どれくらいのお金を渡しは使うべきだと思う」
"on getting a new smartphone?"=「新しいスマホを買うのに?」
ご参考までに。