Doing a clean install of Lion on a system purchased with Snow Leopard results in iPhoto lost. How to recover?
You shouldnt need to buy it as it is included as part of the iLife suite which comes with all Macs. 

From: http://support.apple.com/kb/HT2604 


Insert the Applications installation disc and double-click the Install Bundled Software icon. 
If your computer came with an iLife installation disc, or you purchased a newer version of iLife, insert that disc instead.
Follow the onscreen instructions. Then, in the Installation Type pane, click Customize.
Click the disclosure triangle next to Bundled Applications, select the checkbox next to the application you want to install, then click Install.