Fast Money
There are a couple ways to get bolts faster than going through random levels continuously.
First Method:
Go to the Riglar/Blackwater City race track then follow the video below (or if you prefer text just follow video til on the track.)
When you get on the race track, get to the part where there is a platform that spans the width of the track and is high above it, it should have loads of boxes stacked high on it.
When your there dispatch the enemies just beyond the platform (so they can't interfere) then face the direction you would be racing with the platform just out of camera shot and use the taunter.
Now just stick the button down somehow and return when you want.
(It will take quite some time)
Second Method:
On one planet you have to fight mechs at one point you can go back there and fight them over and over at about 2,500 bolts per go after you defeat them all just jump off the walk way you'll end up back at the enlargement platform as ratchet.
A tip for the fight is don't ignore the helicopters if you make them priority over mechs you might get more than i estimated.
if you decide to vote down please leave a comment so i can improve this guide
(this is not my video)