It's tough for me to tell too. I use more than 2ml per day, mostly because I'm using it evenly all over the top of my head, with a bit more at the front hairline and at the back bald spot. I have my hair really short though, so it's pretty easy to get it all over my scalp.
Using the Kirkland Rogaine that I but at Costco, it's 19.95 for 4 bottles, so even if I'm using a bottle and a half or two bottles a month, that's only 7.50-10 bucks a month. I remember when I tried it years ago for a brief period I was paying 30.00 a bottle, and at that point I was much more careful about only using 2ml a day. Back then I know it used to last me about a month.