besides handling, waste, decurling and trimming, realize that many (most?) of the nicer papers dont come in 13" rolls minimum; they come in 17" rolls. So, if 13x19 is your target, then you immediately get to throw away 4" (although that would yield perfect test prints in 4x6.
work out the price per sq. ft and then factor in the other costs/waste. Also, it's quite likely you're not going to get a nice enough printer that has vacuum hold down so curling and edge curl could cause more waste.
And then there are some paper brands/lines that are more consistently priced than others.
if my calculation is correct, then Canson Baryta in 13x19 is 0.010688 dollars per sq. in but 0.010784 in 17"x50'
You may find it cheaper to buy 17x22 and cut that to sizes like 13x19, 11x17 or 8.5x11 which can also yield a bunch of smaller 4x6 test prints. That at least bypasses the curling issues.
What's your time worth?