The size of grapes from the bunch is similar. (not similar)
If the size of the grapes from the bunch is similar and have a good appearance they are expensive. However, if the size of the grapes varies in size, then you can more likely pay a more reasonable cheaper price for them.
ご質問ありがとうございます。
「ぶどうの粒の大きさが揃ってる」は英語で「The size of grapes from the bunch is similar.」ですが、その反対(不揃い)の場合、「The size of grapes from the bunch is not similar.」と言えます。
また、「粒のサイズが大きく揃ってて見た目の良い房は値段が物凄く高いけど、粒の大きさが不揃い(サイズがバラバラ)の物は比較的手頃な値段で買えますよ」も英訳すれば、「If the size of the grapes from the bunch is similar and have a good appearance they are expensive. However, if the size of the grapes varies in size, then you can more likely pay a more reasonable cheaper price for them.」になります。
ご参考になれば幸いです。