To get a reliable service do I need to sign up and pay ?
Here are the limits:
Public OAuth with API Key
Hourly Cap 2,000 20,000
Daily Cap None 100,000
To better understand the rate limits in the above table, let's use the following example. Suppose you create an application that generates around 3,000 requests from each user per hour. You notice though that users are capped at a maximum of 2,000 calls using the public endpoint, so you register an API Key with Yahoo! to use the authenticated/authorized endpoint. Now your users can make 20,000 requests per hour and up to a total of 100,000 requests per day.
oAuth is free.
References