There are two different avenues to consider when charging money (real or virtual) for your work:
Price it based on the amount of time and effort you put into the work. This is where hourly rates come from. In this case, you might want to determine the “average” amount of time and work you put into X pages of lore, and think of a robux price for that.
Price it based on the final result. This is subjective of course. If your work is high quality and polished, you charge a high price. Rough draft is a lower price. The range of money that you charge depends on your skill level, or often what other people are willing to pay.
In your case I think 100 robux for 20 pages of lore is an extremely low price. If I were you I would charge at least 100 robux per page. Higher if it needs to be proofread and edited afterwards, as that can take hours or days.
Don’t undervalue your work. If you charge a high price and insist that your work is high quality then you will get buyers. But by lowballing like this you make yourself look like a noob. If you believe that your work is good enough, raise the price. Provide examples of your writing for the price in a portfolio.