$25 is all you're likely to achieve from a 'budget't build, absolutely. But the question is, if the build is $475, and you need to make $25 to make it worth your while (debatable)..... why would a "cheapo" buy from you opposed to Walmart et al?
Your 'warranty' idea is a little flawed. Cover your human error within a specified time, absolutely. But don't get involved in components. Either you have to be prepared to replace (out of pocket) a defective component, or you distance yourself fully from the components. It's a slippery slope.
Think about it...
You'd replace a (say) $10 wifi adapter? Seems fine, right? No big deal.
What about a $20 HDD? Still pretty cheap.
A $30 CPU cooler? Still pretty cheap, if you'll cover $20, $30 isn't too much of a stretch.
What about a $40 RAM kit?
What about a $75 CPU (in a budget build?)
$100 motherboard/GPU?
See where I'm going with that?
While the manufacturer's warranty should replace any defects, there's downtime. Something you don't necessarily have from a 'big box' store. A retailers warranty in a lot of situations would replace the product entirely in the event there is a defect, and they would handle it with the manufacturer.
Your proposal would see your customer without their product for X number of weeks while an RMA is completed.....
And at no point did you save them any money. That's a tough sell.
Sure, the higher end builds would net you more money. They'd also be to buyers who may appreciate the RMA aspect a little mor.e................. but the majority of people in that space, can/would build their own.