Magic did have a cost. John was painfully aware of that. Sometimes it took blood, sometimes it took days off his life. Other times, it just took the right incantation and the right ingredients. A sacrifice or so. Either way, magic did not come for free. It had a source, it had power to be drawn from. It took experience and belief and John had more faith and belief than most so called Christians. It was funny given he wasn't religious but he wouldn't deny what he knew was real either.
"If you need me to fix or add any other wards, you'll just owe me something. A favour I can call in when I need it" John replied smoothly. "That's generally how things work. I'm owe enough people back home that I'm surprised they haven't showed up just to call in those favours" he smirked, taking another swig from the flask. He tucked it away and finished his cigarette, carelessly flicking the end away from the memorial.