How much do computers generally cost?

[Yora]

I would assume this happens all the time.

Some smaller shops might sell at almost "production cost" and make the service of assembly and installation nearly for free, to attract and bind customers. But the whole process can easily take an hour per piece for an experienced person, so some extra fee is completely justified.

[Karoht]

So, as a mildly computer illiterate, why doesn't someone make custom computers for half the price of X, tack on 25% for services rendered and sell them to people like me who don't particularly care enough to make one ourselves, like anyone smart would do?

Welcome to my life a decade ago.
You just need an in with a wholesaler, or be a very very savvy shopper and pounce on deals, then markup and sell.

I stopped doing it for 2 reasons.
1-The wholesaler I was dealing with started to be not so keen on my smaller orders and tried to push bulk on me. This means that rather than building a computer custom made for the person, I would have to push certain builds in order to justify buying bulk. Selling certain builds to people who don't want or need them isn't something I wanted to do, and I didn't have a place for parts inventory.

2-Some people who are computer illiterate have a funny way of acting as though you know nothing and they know everything. Some, not all admittedly, but some is enough to increase one's blood pressure.

Between the two, it just didn't seem worth my time anymore, so I went and got a 9-5 job, and do the computer assembly thing once in a while. I 'sell' maybe 4 systems a year, I don't tend to make profit on it anymore either.