I'll put it to you like this. If the cost of the war on drugs is far higher than the cost of increased healthcare due to increased marijuana usage (only the delta between current usage and total usage after legalization applies), you have a net benefit when legalizing. In the case of the US, there is a huge net benefit. Using Canada as an example of this is extremely misleading, they spend nothing on their war on cannabis use.

You also cannot prove that there is an increase in healthcare costs at all, especially because most of the people that would use marijuana if it is legalized already use it. Remember, you used the word increased. Simply being from a "pro-legalization" site means nothing. Maybe they just threw that number out so they could say "see? look how generous we're being with our numbers, and it still costs way less than these legal substances." The fact is that there aren't even any medical journals published that prove marijuana is bad for you more than say, eating at MacDonalds (which is probably worse now that I think about it). The tar from smoking isn't beneficial, but it also isn't bad enough to warrant hospitalizations of any significant number. Also, any negative affects to minors cannot be included because their use would still be illegal. Do you know exactly how many of the people included in your link needed medical attention because of something marijuana use actually caused? Of course you don't. I'm speculating that the majority of any medical cost marijuana might cause has to do with treating dependence. Something you find in myriad other substances, many perfectly legal.

Again, to be absolutely clear, I am not and I have not made a single claim that legalization would have overall negative economic consequences.

Yes you did. You mentioned increased health care costs after the current cost associated with policing the crime of marijuana use. That asserts there is a net cost. In Canada. If law enforcement costs weren't mentioned, you could argue almost anything has a cost associated with it. These grapes made my tummy hurt because I'm allergic. Let's ban grapes.

Oh, and finally. This is from PBS.

For example, a recent Ontario study found that annual health care costs resulting from cannabis use were small ($8 million) when compared to those for tobacco ($1.07 billion ) and alcohol ($442 million).

That is insignificant, especially when you consider how many of them went to the hospital because they thought they were having a heart attack (common for noobs), but actually weren't.