I think you're Dad is getting into an apples and oranges comparison without realizing it. From my learnings, almost all of those pre-fab coops are made with really cheap wood/materials. I'm guessing your Dad is pricing out based on one of the large box stores or lumber yard for materials. The quality of materials he would be using are much, much better than the pre-fab. So while it may look like you're going out to a wash in price, the quality that he could build using the materials he is likely looking at will be much better.
For 6-8 chickens, you're looking at 32 sq. ft. minimum coop size (4 sq. ft. per bird x 8). A lot of those pre-fab ones are giving a chicken count that is woefully too generous. You could almost take their numbers and divide by 3 to get the recommended space count. Meaning if they are saying 12 birds, it's likely closer to holding 4 based on what the suggested norm is here for happy, healthy birds. Cramped birds are unhappy birds and susceptible to illness, etc. So, if you're looking pre-fab for that many birds, you really want to be looking for the largest ones that you can find. Ideally, if you're not seeing them in person, see if you can find out the measurements, and do the math yourself for how big the pre-fab is, and how many birds it could hold based on the 4 sq. ft. guideline.
For me, I purchased a set of green house plans online, and am building an 8x8 coop from that. The greenhouse has 5 large windows for ventilation, is a walk-in stand-up design, and will accommodate about 2x the birds I plan to have. That might be a bit of overkill, but the overwhelming advice that I have been given is to build bigger than you need, and enjoy the extra space in your coop when you have to do your daily chores.
If your Dad - and you - are somewhat handy with building - and I'm going to assume that is the case since you were going to the effort of making out a materials list - you would be able to build something you could be very proud of and most likely, vastly superior to what you could buy pre-fab.
Finally, if you do really go down the pre-fab path, make sure that what you choose can be easily upgraded with better hardware cloth or reinforced in ways that you may need. I get that not everyone can build, so there is certainly no one size fits all here, but know what you're getting into on the pre-fab before you purchase. If you do find a pre-fab that meets a lot of the suggested guidelines that people use here on the site, by all means report back and share! Lots of people could benefit from it, I'm sure. Unfortunately, they just don't seem to come by in the affordable price range that makes fiscal sense (i.e. you could build it for much less).
