Upvote:4
From Tournai to Lille, you can use the train. It takes 30 minutes and costs less than 10 β¬.
Lille airport isn't far from Lille downtown. There is a shuttle from Lille railway station to the Lille Lesquin airport. It takes about 20 minutes and costs 8 β¬.
I believe those are the cheapest options and it still allows you to visit the city as Lille Grand Place is just 10-15 minutes walking distance from the railway stations.