去年5月至今，原料价格走高，若因质量有毛病，二次返工，成本会加大。恳求翻译

可能你的BOSS让你告诉客人 合同现行的价格已经够低了，让你婉言拒绝客户的二次返工要求或者要求返工加价吧。我的翻译是：

As the prices of raw material have been marching higher since last May(楼上的朋友写成了April)，I am afraid it'll take us more to do a further rework. 末了，你还可以加一句pls kindly take our condition into consideration

外贸函电讲究委婉但切中要害，还要避害趋利，所以我帮你省掉了“若因质量有毛病”这句

一家之言 请您参考

The prices of raw material have been increasing since May last year.because of the quality deficit,we must make a second rework,it will increases the cost...

用现在完成进行时表示过去的动作持续到现在,并有可能继续的动作，正好,表示原料价格走高的趋势~!