You can integrate yahoo Boss API key, Google API key, Bing API key, Amazon affiliate key, Ebay affiliate key and Youtube API keys into Inout Search Engine. By integrating Amazon and Ebay affiliate keys, you can earn some revenue. We have developed Inout Search Engine in such a way that it requires minimal API calls to function. Inout Search Engine queries multiple page results in a single API call. So the script will not have to query source engine again for adjacent next pages. Also the DB result caching feature helps to avoid repeated API calls for a same keyword search on a particular engine. Google and Bing provide limited free results, if the limit exceeds you may have to choose some plans to get continued results from them. Google, Yahoo API plans are given below:
Google- 100 search queries per day for free, you will be billed for all additional requests at the rate of $5 per 1000 queries, for up to 10,000 queries per day.
Yahoo - click here to know about the pricing
Bing - 5000 queries per month free click here to know about other plans