if date field is not blank return a number

Hi

I'm trying to create a formula to return a number if a date field is not blank. 

I have 3 fields : Start Date, Cost and Gateway Cost 30%.

The formula is in the Gateway Cost 30% field and needs to be:

"if the Start Date is not blank then return 30% of the Cost "

I tried the formula below in a numeric formula field but it is returning errors.  Any thoughts?

If([Project Start Date]>0,[Cost]*0.3,0)

Thanks

Sarah