SQL - Closest Date match without going over

I have a Pricing file containing prices that has the following structure:
Price
Date
Product ID

I have another file for Products that contains the following pertinent fields:
Product ID
Date_purchased
Date_Purchased_Price (blank)

One product ID can have many price records. The Date_Purchased_Price  needs to be filled in with the Price from the purchase price where the Pricing Date is closest to, without be later than, the Date_purchased.

For example, if the pricing table has the following
Product ID     Date                     Price
234               1/1/2002               1.56
234               3/15/2006              1.75
234               4/10/2006              1.83
234               6/25/2006              1.90
234               7/20/2006              2.25

and the product 234 has a date_purchased of 5/15/2006, the  Date_Purchased_Price  should be filled with 1.83

I can do this within code, but I was wondering if there is a way to do this within SQL.
(We are using Oracle 9i)

Any ideas would be helpful