I've been recently interested in building a computer.

Ok, the way I did it (Way way back in january this year)

I needed a new pc, sick of paying ppl to build for me.
looked into building a pc, learned parts I needed.
built one at a price I could afford. refurbished my old one and sold it to a mate.
Refurbished a couple more units I found myself being 'donated'
Built my Gf a pc for her birthday, same specs as mine.
all the time I was also on Toms a lot, reading,watching posts, learning,
I now build pc's for friends and co-workers, and have a business cleaning pc's in peoples businesses
but yes, you start off building a few to get the idea of system building, and more importantly (Imo) faultfinding, then let people know your building pc's (even just drop it in a conversation as your hobby) and someone within a month will come up and say, 'oh I hear you build pc's', and go from there,
one idea is to have three builds outlined ready, a cheap one for surfing only, a mid one for reasonable gaming and a topline system,
I found that when I gave my list out, the bottom line pc was ignored (people dont often want the 'basic' option, it makes them seem or feel cheap)
and couldn't justify/stretch budget to the topline one, (I dont need a 64 core beast with 12 gfx cards and 128GB of ram  )
so they usually ask about the mid-range one, which sneakily, is the one I wanted to build them all along.
I then tweak that slightly to reflect new gfx cards etc that are out and go from there
I got into this kinda by accident but am loving every new quote and build, I also hold a nightshift job as my main btw so I balance this easily ( its not ready yet,waiting on parts from xxx)  
you give them options, (they dont know what they want most of the time,other than a decent working pc) talk them through it a bit, :
now cost, you can go a few ways on this,
you can show them what you pay for parts and what you add on as your 'fee'
you can just say 'it will cost xxx much'
I show them a system and then say an equivalent system on Dell/cybershower/alienware would cost you XXXXX much, i'll build it for xxxx

but your basic premise of ''You tell me what you want, I'll chuck the stuff in, you pay X for it, and we're all happy assuming my rig works? " is good with an alteration or two imo, YOU tell THEM what they want based on what they need the rig for,
they tell you they want a heavy video editing/modelling system, you dont try to build them a triplecore gaming rig hehe,
and no 'assuming it works' either, theyre paying you to build this, you make sure it works before it leaves your house/workshop/garage, word of mouth will either get you more business, or put you out of business.

other people will posibly chip in with more ideas/tips/help but I'm off shopping with the Gf now, /sigh, chores hehe
Hope this helps you a bit man, but experience is the best teacher imo
Moto