Manufacturing ASICs involves the extremely expensive process of making masks. Depending one what kind of ASIC you want, this can cost up to several million dollars.
You can find more info about that here:
http://electronics.stackexchange.com/questions/7042/how-much-does-it-cost-to-have-a-custom-asic-madeOn top of that, performance mining ASICs are not cheap to design. A lot of time, thought and complex engineering goes into the chip design. Expecting to be able to whip one up within a few months is completely unrealistic.