Is there a way to roundown to the nearest 5,000?

So if its 8,000 - that would be rounded down to 5,000
If its 480,000 - that would be rounded down 450,000

and if its below 5,000 - round down to zero


Any ideas?

I know about the rounddown function but am not sure what to put in there to make it round down to the nearest 5,000?