"I'd say that to anyone who isn't planning to make several versions of their design in order to work out the bugs"
Having read many threads from people that have made their own tent, I have to agree that usually the final version is not the first one...
In several cases the cost of the materials alone ended up more than the purchase cost of a commercially made product. On a positive note, one starts to appreciate the value of a commercially made product.
So I too would suggest that if the intention is to save money, rather than having fun making one, a better choice would be the second hand market or keeping an eye on the clearouts from Spadout and co.
BTW, don't forget to cost all of the "trimmings" , zips,tape,sealant,guylines, grosgrain ribbon, nylon thread, needles for the sewing machine, plus postage and or trips to the shops