I cant recall the landing fee - but it is peanuts.
The GA terminal is about 200 yards from the main terminal. The form is they send a mini bus from the main terminal to take you back to the main terminal to clear customs. Customs can be a bit of an ordeal - usual big airport nonesense, which will go smoothly if the customs man is not having his lunch - otherwise it might take half an hour or so.
Buses run from the main terminal into Lille every half an hour or there is usually a fleet of taxis on standby.
You pay your landing fee in the GA terminal. No problem parking, there is acres of space. If you are affiliated to a flying school you get a discount.
There is one "trick" - there is a gate between the GA terminal and the main terminal with a seperate security guard in attendance. You can usually clear back into the airport through this gate and be scanned rather than get held up with other passengers. Check to see or ask if the gate is manned when you are taken to the main terminal.
Not a lot happens at Lille and I have always had a very cordial reception.
Enjoy.