I strongly suspect that both those reports were mixing up two separate errors: the reason the production budget and the "actual" budget are somewhat different, is because the production budget doesn't include marketing costs. However, there's actually a good reason for this: a *lot* of marketing costs are more virtual than real. For example, when a fast food chain makes a deal with you to include movie tie-in promotions, its very likely that no actual *money* changes hands. Instead, the movie studio "pays" the fast food company in "license to use our trademark imagery" ( which has a theoretical monetary value ), and the fast food company "pays" the movie studio in "advertising".
So, long story short: I don't believe for a minute that Disney actually paid almost 500M to make and sell Avengers. 300M, I'd believe easily, if you figure actual marketing costs rather than de jure marketing costs ( ie, marketing that actually costs real dollars, paid to an independent entity and not another Disney division ).