Here's a better stance:
In truth, you never really know. Get some history behind your landing page so you have a good understanding of its average conversion rates and a comparative metric.
Then for a couple days, throw in the Adsense. If conversion rate drops, see how that compares to the amount you made in Adsense clicks. Maybe conversion rate doesn't drop and you actually are monetizing people who've decided to leave. It's far fetched, but you never know until you just set it on the table and see what happens.
The biggest thing is just to make sure the call to action (your "buy button") is above the fold and prominent.