You'd better find some experts through some channels, so that you can get some accurate answers. It's not easy to answer this question.
It can be said that something is not valuable because it is old. The key is whether it is representative in history. For example, when was it expanded, who was expanded, why was it expanded, and in In times of national crisis, in order to inspire the people to defend the nation and the country, it is better to encourage the people to rise up and resist under alien rule. I think you can understand this.
Also, this rubbing is not complete. How well preserved it is? If you only have dozens of incoherent characters out of more than a hundred words, the value will be greatly reduced.
Then there is the quality of the rubbing, such as whether the paper is good, what kind of grinding is used, etc.
There is another thing that can also increase its value, that is, whether its circulation is clear, that is, whether the process from someone cutting it down to your hands is clear, and whether there are any famous people handling it. After handling it, did they leave any traces on the rubbing, such as a seal or something? Old collectors value this very much. That's it. If I'm not good at it, please correct me.