Still if they can build one it seems to be they should be able to make as many as they want as the real costs for ASIC are development, not manufacturing.
You are thinking this the wrong way. Yes, the cost for ASIC is in development, but the cost to put the stuff in the box sent to customers is in the manufacturing.
Think it in this way. Lets invent some number from my hairy ass.
Lets guess the development of ASIC costs in 100kUSD in internal wages for some competent design people and 200kUSD in NRE for the foundry. Lets also guess to buy and assemble the stuff put to inside each box sent to customers costs 1kUSD. You sell 300 boxes for 1.3kUSD giving you 390kUSD cash in hand. Now what you are going to do:
If you manufacture 300 boxes it costs you 300kUSD. Your total costs are 100+200+300=600kUSD and your revenue is 390kUSD. Net loss of 210kUSD.
If you only manufacture 2 boxes it costs you you 2kUSD. Your total costs are 100+200+2=302kUSD and when your revenue was 390kUSD you get net profit of 88kUSD.
if you do not manufacture anything 2nd batch money is pure profit of 600*1.5kUSD = 900kUSD and with the 88kUSD from batch1 it goes near a cool million.
If you start manufacturing 300+600 boxes your total profit from batches 1 and 2 combined is 90kUSD grand total.
The only way somebody could net a million dollars is not to manufacture more than 2 boxes.
Also there is a fact that very few people enjoy doing the manufacturing. Designing ASICS could be fun for some. Collecting incoming money could be fun. Being arrogant on forums could be fun. Manufacturing a huge number of boxes not so much.