That's the best approach to accumulate Bitcoin but I recommend you to buy only when you see a big dip in Bitcoin's price. The dips occur every once in a while and it will help you to accumulate more Bitcoin. The per month $100 should be more than enough to accumulate some share in Bitcoin network and over time you may be able to save at least a BTC if you continue doing that. I know that at current rates it will take a lot of time to accumulate a whole Bitcoin but I don't think that it's necessary to have a full Bitcoin. We can easily own some Bitcoin chunks and those will be very valuable if the price of Bitcoin grows.