Building your own camera module
There is much else at play in a commercial product other than just parts, cost to manufacture/assemble, cost of packaging, cost of advertising, payroll and facility overheads, repayment of investment development costs and on and on, plus ultimately a reasonable profit margin the market will tolerate. 

Is it difficult to develop your own... well that depends on how much quality of image you want. You can throw the parts together fairly easily, but any graphics device is extremely susceptible to noise interference. It can take many design cycles to get an image that has the sustained quality you would expect.