Help pls: I cannot format a column containing a formula to currency

This is one of those ridiculously simple problems that takes up a disproportionate amount of time.

I am trying to use Numbers to keep a timesheet that calculates the amount to charge a client based on the hours worked on different tasks.

The spreadsheet has 3 columns: Task (Column B), Time (Column C), Amount £ (Column D)

The Amount column is formatted as currency. But when I put in a formula to automate the calculation it changes the formatting to Duration.

I use the formula C[row number]*30 (ie time spent x £30 hourly rate). The column D then changes to show the outcome as duration and I cannot get it to change to Currency. It seems to borrow the formatting from the Column C. Screenshot here:

Column C I have even formatted as text rather than duration and yet Numbers still insists that column D should relate to duration.

Can anyone help? This has already taken up far too much of my time and I'm not happy at being unable to resolve such a simple formatting issue. I thought I'd dip my toe into Numbers but if it can't cope with simple formatting then I'm about to head back to Excel.