I don't calculate the monthly cost. Maybe I used the wrong words to make my point, but at the end of the day, a further out straddle is gonna tie up more capital and it is gonna be a lot slower than the front month so it is still hard to make any money on it from the earnings move.
i forgot to add... many times, there is no opportunity. the impact of the earnings and guidance is not enough to pick a direction that the stock hasn't already reflected by its current price. don't force it by having a personal bias. and your market opinion for the 2-3 weeks matters, too, (if you have any). so if you think the market will be going down, it helps to pick stocks that are gonna go down and don't put a trade if you don't see any. you'll have internal psychological conflicts as you watch the market go in the direction you thought and the stock is being dragged with it, but you picked the other direction based on your assessment of the earnings. good luck.
Some interesting research by a UK Phd student on stock volatility following earnings announcements.......... http://www.efm.bris.ac.uk/economics/working_papers/pdffiles/dp00504.pdf
because its already priced into the option.... as a general rule if it seems plausible that a second grader could think of the strategy you've thought of, the market-makers know how to accurately adjust their spreads to make it unprofitable.