Deposits into mutual funds

[JBW]

I'm trying to work out how to handle my mutual funds in GnuCash. Each 
month I deposit say $200 into a managed mutual fund. So I would want to 
show $200 moving from my Asset:Savings Account to my Asset:Mutual Fund. 
The problem I have is that the Asset:Mutual Fund is measured in units, 
not dollars.
How do I move $200 from my Asset:Savings Account to my Asset:Mutual Fund 
when one is measured in $ and the other in units?