Based on calulations referenced from http://secondlife.com/statistics/economy-data.php for October 7 2010 (file LD_sources_and_sinks):
L$108,331,300 was paid to Premium Residents (PRs) for September stipends (108331300 / 300 per week stipend = 361,104 / 4 weeks = 90,276 PRs for September.
L$27,067,250 was paid (so far) for October (27067250 / 300 per week stipend = 90,224 PRs MTD October...hmm 52 PR's haven't renewed.
So based on an average of 90,250 PRs * US$72 per year for premium status = US$6,498,000 (approx.) generated from PRs subscriptions (not including land use fees), which although unvalidated, is less than the total inworld economy?
I agree with Adam Sparks comment: LL, you made the decision to allow free accounts. This is your service. A free service. I'm all for premium benefits like a pre-built house on a small parcel, a few weekly $L, ect. But please knock off treating basic account holders like a lesser class of resident.
Perhaps the orange 'Join Now >> It's fast, free and easy!' button should include '! denotes for Premium and Concierge Residents only'.