Every now and then, Linden Lab runs a special on new Premium memberships. Past sales have been for the quarterly plan, and you could get your first quarter's membership for $11.25 instead of the usual $22.50.
This month, LL is offering the MONTHLY Premium membership at half off. You can sign up as a Premium member for just $4.98, instead of the usual $9.95. The discount only applies to the first month, and you pay the regular rate after that.
This is a really great way to try out a Premium membership. You get all the benefits, including a free Linden Home, a $L300 weekly stipend payment, access to better technical support and live chat support, and access to some Premium-only locations. If you decide you like it and continue past the first month, you get a $L1000 bonus payment after 45 days.
And if you decide that you REALLY like being a Premium member, you can switch over to the annual plan. That costs $72.00, and if you decide to downgrade back to Basic later, you don't get a refund of unused subscription time. But if you figure it on a monthly basis, it's just $6.00 per month, the best deal going for Premium in Second Life.