1 ml = 100 units
the syringe has 100 units so to get 16 units it is literally only 16% of the syringe. with gh it would only be 1.6 IU and this is why idiots fuck themselves up cause with insulin it is ten times more concentrated per IU
Now with insulin you don't change needle heads from withdrawing, assuming that is what you meant, like pull out with a 21 g and inject with a 23 g, no switching done with slin pins cause you are dealing with 29 and 30 g, just take it out and inject with the same syringe.
as for price, insulin is the cheapest thing in the world, it's like 50 bucks for 1000 units at the very most, that's like 50 cents per day if you are using 10 units a day