One way of doing this is to create a tab in a spreadsheet that has every date that you want in your dataset in a single column. Then you could pull the date field from this new datasource onto your viz and sum up the cost for every day.
Without providing Tableau the dates in between, it basically thinks these dates don't exist.
Are you referring to blending to a date table? If so, how do you blend on a function?
i.e. if I have a date record of 4 Jan, it won't currently line up to any of my dates. It would need some sort of 'between' function...