Yeah, it's 1% per bid.
I too just bid on a player on alt team and existing bid was 709, my bid was 716 and another bid would've been 723. It will never come out exact because the game seems to display values to 3 significant figures but calculates on a more accurate stored value.
I think Becks posted on this surrounding the whole c.2bn max-bit thingummyjig (but I cba looking for it either).