Interesting! I am trying to work out the price plan for my startup as well. Can I ask if you got people signing up for paid accounts directly or they did the free plans first and convert later?
We have a standard 3-6% monthly conversion rate from free -> paid. But, if you look at it in detail, the numbers get more interesting:
About 17-25% of those each month have 5 or more time entries, which is our bar for an "active" account. (A low, sorta primitive bar, but still.)
Of those, 35-55% upgrade to paid :)
Thing is, unless you are in a very similar field with a similar interaction design quality / providing similar value, to people who "get" value for money, my experience on these points probably won't help you.
That's why our sign up page de-emphasizes the free plan regardless, it's just an easier route :)