Many MMORPGs offer a hybrid system. You can pay a monthly fee and have full access, perhaps barring some cosmetic things that everybody has to buy if they want it, or you can purchase things al la carte but play for free. Sometimes there are other restrictions on the free players, such as loot quality that can drop being capped below where it is if you pay for it.
In terms of AH this could be translated as something like:
Free to play, but only have access to a limited number of units/weapons and perks earned at 1/4th normal rate and having to purchase the access to other units and weapons or you could opt to pay a monthly fee as now and have full access and regular perk earning rates.
FTP units might look something like:
A6M5
B5N2
Bf109G-6
C.202
C-47A
*F4U-1C (perk fighter)
G4M1
He111H
Hurricane Mk IId
La-5FN
LVT(A)2
*Mosquito Mk XVI (perk bomber)
P-51B
Panzer IV F
SBD-5
Sd.Kfz. 251
*Sherman VC (perk GV)
Spitfire Mk V
500lb/250kg class bombs
As a starting lineup that hits key points without giving away the farm, you get a P-51, but not the D, a Bf109, not the K-4, a Spitfire, not the XVI. All functions in the game can be done, including something to spend perks on, but in all cases there are better, tempting things to get. Unit costs would probably have to be based on ENY rating or such so a Spitfire Mk I might cost $0.50 and a Spitfire Mk XVI might cost $4.00.
*A strong argument can be made that no perk units should be in the base FTP package, but rather that the temptation to buy something to spend the perk points the player already has earned would be a good way to make more money.
Ultimately, I have no way of knowing the economics of such a system as applied to HTC.