The Medal Year book makes no mention of a difference in price for the GSMs due to RAF/Army/RN.
For the OSM though:
£600-800 Cavalry
£400-500 Line Regts
£300-400 Corps
£400-500 RM
£400-500 RAF
South Atlantic
£700-850 Army (7,000 issued)
£1000-1350 Scots and Welsh Guards
£1750-2500 Para
£650-750 RN (13,000 issued)
£1100-1400 RM (3,700)
£450-550 RFA (2,000)
£500-650 Merchant Navy and Civilians (2,000)
£650-800 Ghurkha
£500-650 RAF (2,000)