How much do you think I need to spend to get a good smartphone?
What kind of price range should I be looking at in order to get a good phone?
この場合、次のような言い方ができますよ。
ーHow much do you think I need to spend to get a good smartphone?
「良い携帯を買うには、どのくらいお金を費やす必要があると思う？」
ーWhat kind of price range should I be looking at in order to get a good phone?
「良い携帯を買うにはどのくらいの価格帯を検討すべきですか？」
ご参考まで！
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。
カナダ 
アメリカ合衆国 
