When your trial subscription expires, so long as you added credit card or other payment information, the subscription will be translated into a standard pay as you go subscription for which you pay normal usage. Spending limits where applicable impose a
spending ceiling and not a fixed monthly commitment to spend.
Unfortunately, normal subscriptions aren't eligible to define a spending limit (http://msdn.microsoft.com/library/azure/dn465781.aspx) as this is limited to trial accounts and special accounts
such as MSDN or Microsoft Partner benefit programmes.
Instead of the spending limit feature, I would suggest you look at the Azure Billing Alert Service which is a preview feature but is really invaluable.
You can read up on how to enable this feature and configure it at
http://richardjgreen.net/microsoft-azure-billing-alert-service/.
I hope this helps you out.
Richard Green | MCSA 2012, MCSE 2003, MCTS Desktop Virtualization, VCP5-DV http://richardjgreen.net
