I'm about to rain my pessimism on you. Sorry.
I realize the limitation due to shipping costs, but I would never pay $10 a month. I'm already one to experiment a fair amount, and I like the idea of being encouraged to get out of my comfort zone with having the grains delivered to make me feel obligated to using them.
I'd be more likely to pay $20 a month for a full medium-low gravity grain bill, with 1 or multiple possible hop schedules and some yeast recommendations (though that is still getting a bit expensive and you'll need to rely on reputation for very good recipes).
Are there any Canadian flat rate shipping equivalents that would make that possible? From quick estimations I'd say that in the states if you could fit a 9-10 lb grain bill in a medium flat rate box (I think this is possible) you would basically break even after bulk grain, repackaging, processing, and shipping prices, though those are quick estimations which mean's you would probably be best charging $25/month, maybe discount it to $275 for a multi-year sub. This also means you are ignoring the possible market of extract/partial mash brewers. If my LHBS is any indication they are the majority of brewers.
Also consider longevity/cycles. Even if you were able to get the specialty grains down to a reasonable price per pound, I'd be pissed if one month I paid $5 for a pound of crystal or roasted barley with a recipe for a basic amber or stout. I just ran though the list of grains my LHBS carries and there are about 5 or 6 individual grains that I think could be interesting for your initial idea, meaning a customer that after 1 year would not be back.