Well, it's $3 for a sachet here and if you use it every time to incubate a new batch then it isn't cheap...
I think I'll be doing like musi is doing... I'll check how my 2nd yogurt turns out.

Six envelopes for 3.95:
http://www.scdkitchen.com/yogourmet.html#starter


..and yes, there are early active cultures inside the packets which die out as the PH changes to acid.

YOU SHOULD NOT USE SCD to start another batch.

Store bought is fine since it isn't sour therefore the early critters are still okay.

If you really cant afford it then use a single envelope for many liters of SCD. Drop the temperature of the warm milk to 26c add the envelope and then let the Multi http://www.scdkitchen.com/yogourmet.html
bring it up to ~40c, the time that takes is enough to allow the early active cultures to bloom.
But the 24hr period does not start until up to temp.

Two things, make sure your room is 26c or less and either use a Multi or build something that will bring up the temperature in a timely manner.

Fermentation Box
I use the Multi now because its so easy but a cooler with a heating pad works fine. For those with a bit of handyman in them you can get accurate shut off thermostats with a temperature sensing bulb on ebay for <$15.
NOTE: Make sure its accurate to less than 1c.

Buy a plastic electrical box and an extention cord, set the Thermostat to 41c, mount in box. Attach the extension cord as a power supply add a standard plug and cover to the box and bingo. You can now put your yogurt into the cooler, plug in your heating pad into the box you just made, put the temperature sensor bulb into the cooler and plug in the extension..

NOTE: You can use this method inside an oven or wherever but a cooler allows you to cook and bake while fermenting.

NOTE2: If you also make mead you can use this with a different temp setting to make it in winter..that said there are already plastic buckets available with a heat band running on the outside for this.

NOTE3: Or just steal some yogurt at the 4-6hr mark to use as starter the next time..