You pay a Monthly 100$ btc.
You receive  in a month 70$ value of alt coins.
And at the of the month you get like 10$ back of your btc.

At the End of the month you hold different altcoins and you hold little bit bitcoins.
Now if you pay for the next month 100$ you pay 10$ less so coupond that  by  12 months you have profit of 120$ btc and a lot of altcoins.