Since marijuana is a non-essential, of course the optimum amount of money to spend on it is $0, but I don't think it's realistic to expect you to go from how much you're smoking to nothing all of a sudden. Nothing habituated dies that easily without a lot of external support.
It's hard to say how much is a frugal amount to spend on a "vice." Let's say we're using the MMM beer numbers. A six-pack of quality beer where I live is $10 (it will be less for you I'm sure, as I live in a HCOL area). That's about $40 a month. An 1/8 of quality Devil's Lettuce in my area is $60 IIRC. So I'd say if you're smoking a little under an 1/8 a month, that's not bad.
I know that's a lot less than you smoke now, but I also think you're getting very diminished returns on the quantity you're smoking. Your body has certainly built up a major tolerance, so a lot of what you're consuming is wasted.
I think I remember from your other thread that one of the obstacles you're facing with cutting back is that your partner is also a prodigious smoker. Have you had a conversation with him about your desire to cut back and curtail your spending?