C checks cost 60 times an A-check, regardless of plane size, and regardless of a small C check lasting 7 days, and a very large C check lasting 20. D checks cost 300 times an A-check, regardless of plane size & check length. I think that could be better balanced. Maybe SM/MED/L/VL C-checks costing 30/45/60/75 times an A-check, and SM/MED/L/VL D-checks costing 150/225/300/375 times an A-check.
This would drop the cost of maintaining small/medium planes from what it is currently, leave large planes the same, and slightly increase the cost of maintaining very large planes. Makes sense to me that if two planes have the same a-check price, a 7 day c-check should cost significantly less than a 20 day c-check.