IMO there is not really a single best way to purchase corals and online subscriptions for automatic delivery is probably not going to save you any money.
I’m reefing on a budget and have found the best way to get the nicest corals for the best price is lots of diligence, patience and research.. I have most of the online retailers book marked and I visit their sites frequently looking for sales and bargains, I know what things should cost and when I find a real bargain I will jump on it. Local Reefers and you LFS can also be a source but again it’s just watching and waiting for that great deal on something your looking for.
Local clubs can be a great source also.. mine has a huge giveaway once a year and they literally give away a thousand or more frags to members.