It looks like all those API use this "standard" of 10 recent transactions. OP will have to make 10 requests and merge them together to get 100. He might need more than 1 request per minute to get more precision...
It's not like the paid API is unavailable, an API key to get a few thousand unlimited calls is dirt cheap (we're talking like $10), the only problem with it is that their payment processor only takes Paypal which excludes pretty much everyone who doesn't have it.