https://www.goodrx.com/humalog
$68.38 for five 3ml cartridges of 100 units/ml.
So if you need 100 units/day (that's on the high end), that's two weeks worth.
You also need a basal insulin for Type-1 diabetes (unless you're on an insulin pump, which is an entirely different class of expense), so something like Tresiba ($505) (https://www.goodrx.com/tresiba) or Lantus ($357).
Still not $1300/mo, but your price is also using temporary rebates and coupons, which certainly aren't always available.
$250 for five 3ml at 100 units/ml (1500 units total).
$324 for three 1.5mL at 300 units/ml (1350 units total).
$357 for five 3ml at 100 units/ml (1500 units total).