长期以来我们一直卖这个价 怎么说啊?

一楼回答得不错噢。不过应该是
We always sell the product at this price since a long time ago.
这里要用ago。before用于过去完成时。

长期以来我们一直卖这个价

We always sell this price since long year ago.

We always sell the product at this price since a long time before.

It was a fixed selling price for a long time.

We've been selling the product at this price for quite a time.