My engineering school in France uses Minna no Nihongo, not Genki, but since they’re both well-known beginners’ textbooks, I guess I can try to give you some numbers to work with.
I don’t do MnN lessons myself (I’m in the advanced group within the class), but I know my classmates who are doing MnN seem to do 6 lessons per year with 1.5h of classes a week? Each of the two volumes of MnN contains 25 lessons, so one semester only covers a quarter of one volume.
Assuming Genki’s and MnN’s contents are comparable, I’d say that whatever you get at the Japan Society is definitely significantly faster. To put it another way, let’s do a quick calculation (tell me if I’m getting the figures wrong):
5 x 5 weeks = 25 weeks to finish Genki I
That’s, what, 6 months? Never mind Japanese: from what I vaguely remember, most French courses (and I mention French not just because I speak it, but because it’s more similar to English than Japanese is) from well-known French teaching organisations like the Alliance Française will take at least a year to finish a beginners’ textbook unless you take intensive classes. 6 months is fast for a group course. (Obviously, private lessons can be faster, and so can self-studying, but that depends on the individual as well.)
Still, whether or not that speed is worth the price is up to you to determine. I personally would not pay that much for a course that revolves around a beginners’ textbook, but I also have a lot of experience self-studying various fields and skills (Japanese is one of them), so perhaps I have an easier time working alone.
PS: If you are tempted by the self-studying option though… might I recommend the textbook I used, which is also available as an e-course? It’s called Japanese with Ease, and is roughly equivalent to Genki I & II or MnN 1 & 2. I finished it in 7-8 months, and it only took that long because I had other commitments on the side. Here’s a link:
It’s possible to download trial lessons from the textbook (with audio) in order to see if it suits you, and it’s a course that’s designed for self-study – no teacher needed. Of course, I’m not saying you should do this instead, but I just want you to know it’s an option (and it also happens to be much cheaper than what you’d pay for any classes covering the same content).