it's intended that the production price is so high
I think it would be more reasonable to limit the production to alien and
newly researched items, aircraft-shields etc.
Basic human items like old-fashioned weapons etc. should be available
in big enough quantities on the market.
How about filling up the amount of offered items on the market
each month with a certain amount, say half the initial market,
or half the number of items bought in the last 2 month ?
And the player will need some more than offered on the current market,
just to outfit the soldiers in base#2 .. base#8.