My computer was $499 and monitor was $149, or $648 altogether.
"All together" means everyone or everything together.
The dogs started barking all together.
When the starting pistol goes off, we will begin the race all together.
I often find the term "all together" being awkward. If you can split the term the sentence usually makes more sense and validates the use of all together in the first place.
All the dogs started barking together.
When the starting pistol goes off, we will all begin the race together.