The libraries were the ones that came to mind as well in terms of what is available for free.
You could try either Memorial Arena Meeting Room, Nassagawaya Tennis Center and Community Hall board room (regular rental rate are about $24/hr), or Board Room 1 at the Sports Center ($33/hr). Not cheap but the smallest and least expensive available municipally. You can get 50% discounts if rooms are available and you book no more than 48 hours in advance (a fair likelihood on a Tuesday at 10AM)
I'm assuming it needs to be a private room otherwise I'd suggest Hawthorne Cafe, Mama Mila's or Hen & Rooster as options.
Maybe try calling the Milton Chamber of Commerce - they might be able to point you to a place.
The MEV Innovation Center sounds like a great option too - but boy, expensive!
http://milton.ca/en/townhall/facilityre ... _mid_=6887