Hey there, don't know if it's really *THE HIGHEST*. But in playthrough 1 I never found one with more than 500. So I think my 48x-Damage sniper (ultra rare white), should be somewhere on the top.
But in playthrough 2 and 3 they can reach up to 1000.
Some say that the best sniper in the game is the Jakobs Skullmasher - in 2nd PT I came across one with 682x5 damage. On a critcal hit the damage from this can go over 9000. *snigger*. Seriously though, the highest damage shot I have seen was 99,999 damage from a Skullmasher onto the Destroyer.
Yup, that is the highest the numbers can go but I think, just like the money, you can go higher however you will not see the actual number more than 99,999. The only way you are going to know is if you can kill big mobs in like three hits. Oh, and yes, the Skullmasher is the best in the game over all, but like all the other guns it depends on what you are looking for and in what order you want.