In the last transfer window there was one sale made by a League One club for over £1m (only three over £500k) - that was Jonjo Shelvey to Premier League Liverpool where he has already played several times in Europe and the league and cup.
As far as our own track record goes, we have sold a grand total of two players for more than a million in the last 20 years - Bobby Zamora (England international) and Adam Virgo (the sale of the century - thank you Gordon Strachan!)