The best chance of succeeding in this I would guess is to get funding and scale quickly. I've seen people attack this area before and they basically run out of money and the project hangs. You need to project a lot of confidence and stability while moving fast - so adequate funding will be key.
Good luck, if you nail this hundreds of SaaS startups will be very grateful.
PS: pricing is way too generous, think in terms of revenue.
So a SaaS with 1000 users is getting something like $20K gross a month, with 10K users that's $200K gross.
I'd change your levels to something like
100, 500, 1000, 5000, Contact Us