If you want to legally own the entire franchise on Steam/GOG, you will spend approximately $80-$100 USD (8 games x $8 + $15 for XX). This is expensive for games that are 20+ years old. However, during Steam sales, these titles routinely drop to $3.99 each.
Metal Slug 6 is the trickiest to get. It is not sold standalone easily on PC. To get Metal Slug 6 legally on PC, you must purchase (via PlayStation Store on PC cross-buy, OR the defunct SNK Arcade Classics Vol. 1). Alternatively, Metal Slug 6 is fully playable via the SNK 40th Anniversary Collection on PC, which includes the Atomiswave version. Metal Slug Collection 1-2-3-4-5-6-7-X PC Comple...