The Destiny 2 base game without all the extra DLCs will take somewhere around 12 hours to complete. It is definitely worth the low price tag just to run through the story missions. The DLCs usually add around 5 hours of extra story missions. After that it will just be grinding strikes and doing raids with your buddies to try and get those coveted weapons and armors you seek.
With the lull in content right now, I have seen the Destiny 2 base game for PC, Xbox, and PS4 be as low as $10.00usd. I really think its worth to pick it up and try it out before you drop a bunch of money on the season passes.