I can't say for certain what I'd do in that instance. I was faced with a similar situation not too long ago. I look out for a number of things for some friends of mine. One time, I found a Justice League Hawkgirl, and I knew a friend was looking for it. So, I pick up that and something else, and head to the registers at my friendly neighborhood WalMart. The cashier tries to ring Hawkgirl up, but it isn't scanning. So, she doesn't even ask me a price, and just enters it at around $4.99. I know something about toy lines, and that this figure should sell for about $6.99 or so. But, she did this so fast, I didn't even have time to protest.
In my opinion, you should try to be honest as often as possible. I think in the long run, it will pay off in karmic value. Honesty will pay off more often than dishonesty.