Well, I figured out that it's 1 million bolts cumulatively (luckily), meaning that all bolts spent still count towards the overall goal (no need to be penny-pinching ammo, or taking strategies that limit buying weapons for example).
Taking this into account, after story, weapon, R.Y.N.O and gold weapon purchases, you will only need a further 350k-450k bolts to earn the achievement.
Now, There's two ways of doing so:
Complete the game & clear the levels over and over until you earn the required amount of bolts. Definitely a grindy strategy, but each level gives you roughly 10-15k bolts, so it shouldn't take too long.
However, don't bother 're-completing' worlds in a playthrough (unless needed by the story), as running through them again, you will notice the vastly reduced enemies & bolt crates - you'll barely earn anything doing it this way. Just stick to following the story as much as possible.
At the point of writing the question, I was at about 275k, and halfway through the 4th playthrough of the game (after collecting everything as stated). So it isn't an impossible feat, but one that requires a lot of time.
There is a wall glitch that allows you to get onto the Hoverboard track whilst walking in Blackwater City (Rilgar), with full control over your weapons. The bolt boxes respawn there as soon as they drop out of frame. In other words, if they aren't on screen, they'll respawn.
To use this glitch, you'll need two specific weapons: The Decoy Glove and the Taunter. The Decoy Glove will aid in pushing you through the wall, whilst the Taunter's ability to break blocks will come in handy later.
Using the wall glitch:
- Behind the alien that launches the hoverboard race there's the button elevator you originally came up from to finish the level.
Just to the right of this elevator the walls form a 'V' shape, seen here:
Standing in the V-shape and aiming the Decoy Glove at your feet, you can 'push' yourself through the wall. it may take a couple of tries (and a couple of decoys) to get through. Note that you can also use the Gold Decoy Glove, it makes no difference.
- Once through, follow the curved wall around until you can walk no further. Do note that one wall you can pass through, it's just a bit beyond that.
Jump & glide down towards the water, aiming up and left of the Nanobot boxes:
From there, sprint-jump across to the hoverboard track.
Using the Taunter
- Get to the part of the track with the large jump into the suspended, stacked boxes. Kill all the Amoeboids nearby. They won't chase you, but it's easier to setup if they're gone.
- Stand just past the suspended boxes, facing the camera, and use the Taunter. If you hear breaking boxes and see bolts start flying toward you, you're doing it correctly.
- Rig your controller to press 'fire' as long as necessary. It takes a while, but eventually the achievement will unlock.
Here's a handy guide that shows the entire process: