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Here's Apple's official AppleScript reference:

I actually find Matt Neuberg's book AppleScript:The Definitive Guide (O'Reilly) more helpful.

Here's an example of an AppleScript that does approximately what I described above:
tell application "OmniPlan"
	repeat with t in tasks of front document
		if task type of t is standard task then
			-- effort is in seconds
			set budgetHours to (effort of t) / 3600
			set budgetPrice to value of custom data entry "Budget Hour Price" of t
			if budgetHours is missing value or budgetPrice is missing value then
				set budgetTotal to 0
				set budgetTotal to budgetHours * budgetPrice
			end if
			set value of custom data entry "Budget Total" of t to budgetTotal
		end if
	end repeat
end tell