I'm curious about this as well and was going to submit this question to HN. I'm debating between charging say per $5 per user or like you say a tiered approach:

$7 - 1

$12 - 5

$22 - 20

$40 - 20

$90 - 50

$150 - 100

On one hand a flat fee per users seems to be the simplest approach where as the tiered system implies a deal with more users.

I'm leaning towards the tiered method. Everyone loves to feel like they are getting a deal. I know I do and when I put myself in the buyers shoes I think this is what I'd like to see.