it all boils down to modernization budget and maintenance of any purchased equipment. To give you some Ideas on how much is the cost per flight hour, I copied and pasted some date from another forum (timawa.net)
MRF
F-16: Between $3,600 [1] to $9,000 [2] per flight hour
F-5E: $3,910 per flight hour [3]
F-5A/B: P114,660 per flight hour (1999 prices) [15]
F-8H/P: P92,690 per flight hour (1999 prices) [16]
F-18A: $5,977 per flight hour [4]
F/A-18C: $3,871 per flight hour [7]
Gripen: $2,000 [5] to $3,000 [21] [22]
Mirage 2000: $2,700 per flight hour [6]
Mig 21: $4,500 per flight hour (Lease rate, therefore includes profit mark-up) [8]
Mig 17: $2,300 per flight hour (Lease rate, therefore includes profit mark-up) [9]
F-4 Phantom: $5,628.52 to 5,804.41 per flight hour (See here to determine how these figures were computed)
F-15: $6,000 to $8,000 per flight hour [13]
Su-30MK: $12,000 per flight hour (RTAF) [19] [20]
F/A-22: $44,000 per flight hour [21]