I know what you mean, these systems have been supplied by customers, or when I am approached to do an install they say "I have seen an 8 camera system on eBay for £400 with a monitor"... that kind of thing. The problem is that it is so easy for people to search for systems and get very cheap products they can't understand why I want to charge them double for materials.
A lot of my customers are on a budget, so I normally start the conversation with a disclaimer to state that I do not recommend the products they want me to install and therefore can't give them a warranty. If I do an install in one day and make say £250 out of it and it doesn't break the bank for the customer plus they have a reasonable system installed to the spec they stated, plus I have a disclaimer in place, that's the only way I proceed for a bottom end system.
For prosumer standards I go for 600 TVL Sony CCD cameras, RG59 throughout, a decent monitor and an XDH DVR or similar.
Plus I have to explain the pitfalls of the quality of their internet connection and how this can affect remote viewing. I have fallen fowl of domestic gear in professional situations and learnt the hard way! Still, you live and learn!