Same subscription for multiple stores?

I have multiple stores in different sites and I have installed Discounted Upsells in one of them. Can I use the same subscription and install it in all of my stores?

No, this is not possible. Each store is charged separately for each app that you install.

The price is higher compared to what I am paying every month on my first store. Is it possible to match the prices in both stores?

The pricing of the plans for this app is actually based on your Shopify plan, so a price match is not possible if you have different Shopify plans on both stores.