When you die in Borderlands, you keep all your items, but lose some cash when you respawn at a New-U station. I can't find a clear answers of what percentage of your cash is lost or if it scales with some other factor as well.
Borderlands – How much cash do you lose when you die in Borderlands
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The amount of money you lose upon death is approximately 7% of the total amount you're currently carrying. While there is some variability on the Internet about what this % is (5%, 7%, 10%, etc.) the top voted answer here seems trustworthy as the user ran 3 separate tests all in a row.