Belgian Railways has just published information on its 2013 "summer deal", which starts on 24 June 2013 (ends on 31 August) and costs €7.50 single, €15 return - the same as last year. Between those dates, that is therefore the maximum you need to pay to travel from/to anywhere in Belgium, EXCEPT Brussels Airport which requires a "Diabolo ticket" on top of any flat-fare ticket. As this costs €4.44, total cost using the summer deal fare would be €11.94: so it will still be cheaper to use standard single tickets from BRU into e.g. Brussels (€7.80), Leuven (€8.10) or Antwerp (€10.40): but not Ghent or Brugge.
With respect to Charleroi Airport, on the other hand, note that it costs €5 to take TEC bus A to Charleroi-Sud station so paying €7.50 for the train from there to BRUSSELS makes separate tickets on bus+train cheaper than the one-way shuttle fare at €13 (although not the return price of €11 each way); and much cheaper than the combo ticket price to BRUGGE, €20.10.
It also means there is no need to consider/use the Belgian Railways 10-trip Rail Pass, costing€76, for trips between those dates.