by the time you rank up to general, u will have all the base helmets so no need to worry, overall u just need 2,000,000 credits
in total there are 19helmets (including the default helmet which is already paid for from the start) which i follow with a list of all the BASE helmets below (without attachments)
MARK V[B]
CQC
ODST
HAZOP
EOD
OPERATOR
GRENADIER
AIR ASSAULT
SCOUT
EVA
JFO
COMMANDO
EVA [C](emile's skullface version of the eva helmet)
MJOLNIR MK. V (Master chief's original helmet)
PILOT (similar to EVA)
RECON
MJOLNIR MK. VI (Halo 3 default helmet)
GUNGNIR
SECURITY
and now the two waypoint helmets which do not matter, to get the haunted helmet
MILITARY POLICE
CQB
[Edited on 10.04.2011 9:00 AM PDT]