This will, of course, vary on how many servers someone plays on. One account with cities on all 8 servers, that all get Silver every week, is more diamonds than 3 cities all getting Gold. But I would not be able to do that, because I would want to play all my cities fully and use diamonds in all of them. That is part of why I cut down to just one US city. lol
And yeah, I think I spend $5 every time the 100% offer comes around, so $20-25 per year? Silver every week in one city, at 125 diamonds guaranteed, is 6500 diamonds in a year.