Hi,
First, please deactivate the free version of the plugin and download and install the pro version from here. Secondly, you will need to install the WP Factory Helper plugin and add the license key there, to receive automatic updates.
To set different costs on the same product you’ll need our “Products > Add stock” option. After activating it you will find a new meta box on the admin product page with two new fields: Stock and Cost. Following your example, let’s say your price for 8 pieces is 100$. In that case, you’d enter 8 in “Stock” and 100 in “Cost”. The next time, it would be 3 and 70. In the end, your new cost will be the average between the costs. Something around 92$ in that example.
Just a note, before entering the values, you’ll also have to enable the “Enable stock management at product level“.
For your third question, you need to multiply the cost by the stock? If that’s the case, please let me know if the reports present in WooCommerce > Reports > Stock > Cost of goods can help you?