(2-part post):
I thought that things were getting rather expensive as well!
Yes, those fare quoted are
single fares. So you're looking at paying over �70 for the return journey. Paris is over 200 miles from Calais by train and the TGV trains aren't cheap.
My advice would be to leave Paris for another day. If you book far enough ahead you can travel for �59 return, on Eurostar, from St Pancras, Ebbsfleet or Ashford. (I've done that plenty of times so I can provide plenty of suggestions if you need them. About 18 months ago, I even met up at Waterloo with another ABer, who was visiting the UK from Colorado, to give him a personal guided tour of Paris!).
So that leaves the question of where to go on the 5th April. To me, the answer is obvious.
Lille is a stunningly beautiful city. If you take the 0643 train from Calais Ville, you'll be there at 0802. Coming back, the 1821 train gets you back to Calais Ville at 1940, so you can still get a meal at Au Calice (or your own choice of restaurant), as above, before heading for the ferry.
The fare from Calais Ville to Lille Flandres is �12 each way. Pre-booking is not required because it's a TER (regional) train, rather than a TGV service. (NB: When heading back to the station in Lille, ensure that you go to Liiles Flandres station, not Lille Europe, which is the Eurostar station).
Because I've only visited Lille via Eurostar, I can't give you directions from Liiles Flandres station into the old town area but there's a good bus tram and metro network. A single journey costs �1.25 or you can buy a day pass for just �3.50. (Alternatively, you get free travel with the City Pass. See below). For a map, click the second link here:
http://www.transpole.fr/c/c_a_a_acrobat.aspx