Thalys is a joint service offered by the belgian, french, dutch and german railways.
You can find the different fares available here:
http://www.thalys.com/fr/en/fares
And, for a PNO/BMI trip, the list prices are:
http://www.thalys.com/fr/en/fares?s=...oStation=BEBMI
Code:
(Prices in Euro)
Librys Mezzo Mezzo+ Smilys Kid Kid & CO Youth Senior
Single Return Return Return Single Single Single Single
Ticket Ticket Ticket Ticket Ticket Ticket Ticket Ticket
Comfort 1 122 182 134 - 15 61 61 85
Comfort 2 78 116 85 54 15 39 38,50 54
If there is a slight difference with your quoted prices, it is mostly likely due to (what else?) taxes.
(depending on what you want to do in Belgium, consider buying a "Any Belgian Station Ticket"
http://www.thalys.com/fr/en/any-belgian-station)
While I have traveled a lot on the French TGV (High Speed Trains), I traveled only once on a Thalys (PNO/BMI), a long time ago.
Yet, I distinctly remember the trip for being too short! I was comfortably seated and it seemed that I barely had time to relax before the train pulled in Brussels station...
IIRC, at the time, I had the Youth rate. Milepig, who paid 46 euros for a return ticket, probably had a "Smilys" fare.
Guy Betsy, unless you can book in advance, you probably won't find better than 78 euros.
hth,
GT