It's a costly operation. Generally, if you're forced to make the heads move, you're talking 5+ milliseconds (and the 5ms is probably a high-end drive)

Technically, you're only playing the price when reading (or writing) after the seek. And then only if the requested page isn't already in the buffer. The seek itself doesn't move the heads. And you may have to pay said price anyway, even without seeking. The heads may stay at the same spot if your process is the only one accessing the disk.