United turned down a £1.5 million offer from Sheffield Wednesday for the 21-year-old forward earlier in the week and confirmed that Preston were among the clubs interested.
However, Simpson feels the Iron are demanding too high a fee for the 32-goal top scorer.
"I think everybody is interested in Billy Sharp," Simpson told Preston's official website.
Preston boss: Sharp too expensive
"We made an enquiry at the start of last season and the start of the year but the prices that they were quoting were too much and I actually think the figure that they are talking about now is as well.
"We have just got to keep looking around until we find the right people to try and make the club better."