You have to go right through to the ticket selection process and select The Smiler fastrack before you find the price. This link
should take you to it.
Personally I think £10 for a single fastrack for just one ride is too much, even if The Smiler is the latest addition to the park. A Fastrack Scream for 3 rides is just £2 more at £12. I don't normally use Fastrack, if there are large queues I tend to use single rider, typically it's much quicker than the main queue (although you have to judge it as sometimes single rider does take longer). However, as Themeparksandy1981 said, there is usually someone willing to pay to skip a 2-4 hour queue.
It will be interesting to see if this price remains for the season, or if it reverts to the £7 it was for a single Smiler fastrack last year.