Here are the details on pricing:
https://aws.amazon.com/lambda/pricing/
You get up to 1M requests / month and 400,000 GB-seconds of compute time per month.
A default lambda function uses 128 MB of ram (0.125 GB), which gives you about 3.2 M seconds of compute time (time actually spent executing requests) every month for free.
Thus if you have functions that take 500 ms on average, and use the default amount of RAM, you can process about 6.4 M requests in a month for a total bill of $1.20.
Above the free tier limits you pay $0.20 per million requests, and $0.1667 per million GB-seconds.
The pricing is fairly attractive.