This might help:
substitute "mybuild\$(?...)" with "$(BuildNumber)"
I had a similar problem where I was trying to copy from the drop location to a "Latest" folder. I found that to construct the final destination of the files (\MyServer\MyShare\builds\MyBuild\Daily_20090708.14\Mixed Platforms\Release) translated to the following using variables:
$(DropLocation)\$(BuildNumber)\%(ConfigurationToBuild.PlatformToBuild)\%(ConfigurationToBuild.FlavorToBuild)
I also found it helpful to use the Message task for troubleshooting. The following task will tell you what the variable translate to.
To see the output, look in BuildLog.txt.
PS: My tasks that used the properties and items mentioned above where located inside .