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It's all cheaper than a DME+ stirplate. Boiling sugar with some acid(lemon juice) breaks the sucrose into fructose and glucose so the yeast can eat it easier. The baby food is added nutrients to keep the yeast healthy, shaking the soda bottle oxygenates it. I have done this for the past 10+ years and never felt that a stirplate would so much of an improvement that it would be worth the cost.
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You are only supposed to use maltose to grow yeast, as the successive yeast cells will grow lazy with the simple sugars and will not perform like you want them to in your eventual (maltose) fermentation. Now if your are making wine or something you might be fine, I don't know anything about repitching on those fermentations.
I bought 5 lbs of DME and 1 container of wyeast nutrient and they last me a LONG time. (and I make a lot of starters, including lagers that need decanting and stepped. If you want to be real cheap, save some runnings or just make a separate mash and freeze until you need it, then boil to sanitize.
ETA: I don't use a stirplate, just a 1 gallon glass jug.