Many of the SSO products have very complex pricing schemes, involving a series of optional add-ons and other charges. But typically they charge somewhere between $2-$10 per user per month for their features. Typically, you get the best deals if you purchase an annual contract, and support is usually an extra fee.
A few providers have a trial account for a few users or a few apps. OneLogin is
impressive in that theirs includes unlimited users for 8 apps for free. PerfectCloud has a free plan for individual use, and 15 day trials for their enterprise plans.
Here are links to some of the vendors who have more transparent pricing pages: