I'm in complete agree with all that, well put.
The reason I originally posted was that I didn't feel easy about:
I wouldn't find that easy or justifiable, and I would certainly worry that it could easily turn round and bite me at some point in the future.
No of course not, but that wasn't the scenario. That 'someone' could also have decided to build the site himself, or decided to go with a bloke he met down the pub who is a bit handy with all that internet stuff and once built a web page that seemed to work ok. But instead he decided to hire someone whose job it is to provide a professional well-tailored package.
That's where your expertise comes in, finding out about and understanding the needs of his business, factoring in growth and scaleability. If he says he has seen a ready-made 'solution' for the store part of his site for $200, then either you think about it and agree that that solution may be appropriate for his situation or you don't. If you do, then you say that you will implement it at a cost. If you don't then you can explain exactly why it would not be appropriate and why a custom solution would benefit him more. You have the facts, the experience and the skills, which is why he has arrived at your doorstep.
Yes, people have perceptions, but you have experience. Pitch perception against experience, and if you are pitching informatively and with the client's interest in mind then your experience should inform his perception. For those clients that just won't 'get it' and are mesmerised by the dollar amount at the expense of what's best for their business, then your problem already extends beyond just PowerStore v. Custom build.
I couldn't agree more.