It won't have much effect on the buy rate because Undertaker has never been a main attraction at the top of the card. Undertaker and the undefeated Wrestlemania streak is actually a fine example of how well the WWE hype machine works. Not to downplay Taker's legacy or the positive (or negative) effect that breaking the streak could have on someone's career, but when it comes to trying to appeal to the masses and the non-wrestling fans, Taker and the streak aren't really what you'd rely on to draw in the non-wrestling fans, even for just one night to order WM. Wrestlemania 26 proved that. That said, I'd love to see Undertaker compete at Wrestlemania. He's had the best matches the last 4-5 years.
I don't doubt this Wrestlemania could top last year's buy rate, given not only Rock's mainstream appeal like last year, but also Brock's as well.