I'd be completely unsurprised if it was already in the tens of billions, tbh, just from the flooding that's already occurred. Houston effectively has rivers where some of its major roads were, right now, and there's more than houston that's been taking this to the face.
... and the amount of rain predicted to come down by the end of it is from around 50% more to outright doubled or greater what's dropped since landfall. What's hit so far is basically only the halfway point water wise, and floods... tend to have breakpoints, where shit goes from just disastrous to catastrophic. Like, say, the point when reservoirs and basins and whatnot start overflowing, or have to have water let out to
stop full overflow from happening. And,
well. Folks overseeing those things have said outright they're going to be letting some loose, and it's going to almost certainly cause shit to get wrecked elsewhere. Just... less than it would if they didn't.
Also real noticeable on that news release is the mention of retaining water in those reservoirs -- or from what I understand, i.e. conditions are still flooded enough the ones specifically used for flood control are needed -- for a period in the range of 1-3
months.
E: Though a bit of poking around suggests it's more that they expect it to take that long to safely drain out. Still kinda' yikes, but a lesser sort.