A point missed?
The headline cost of an instalation from this company might well be higher but this will include all the interconections with other kit and a quality job.
Other quotes usualy are only for instalation of the "box", quality of workmanship is very hard to judge but on the whole they are very good.
So the issue here is "what are you getting for your money" and compairing like with like.
I would guess that my aircraft are flying three or four times the hours that you are and reliability is a real issue (aircraft on the ground are not making money) so long term reliability is a paramount.
I think in the long term this company are the cheapest option for my company as I know that if they do the instalation I will get years of trouble free service from the job.