Actually, 100,000 requests per month is a little less than a req per second. Which means that if you want to have all the services that GH is offering, I guess you would pay quite a lot (relatively speaking).
There are 86400 seconds in a day. At a little under a request per second, you're going to hit 100k some time around 3am on the second day, depending on what you take "a little under" to mean.