I go over to France from Plymouth /Roscoff a lot......its a very good deal for a day out although having a house 40mins from Roscoff I go for longer most often.
If you go on a cheap day....
do not and i repeat do not buy anything anywhere near the port....its always expensive even when the euro/pound is good!
Go down to Morlaix and try the local shops or the fantastic market on a Saturday. If you want some food or wine bargains try the E Leclerc supermarket at St Pol de Leon.....and the Lidls opposite it
.....both signposted from the road out of Roscoff to Morlaix.
You an still get some really good wines for 2 or 3 euros a bottle, and less if you buy a box. There are some great bargains in the chese counter.........much less on some varieties such as morbiers and brie than at home and as for chocolate spread and posh jam, not to mention mustard....well thats still cheaper too! I still think you can fill the car and save the ferry fare on a cheap day return!
And whilst you are there a meal in the blue creperie in Roscoff....cannot remember its name.....is very cheap and better value than eating on the boat!