From the yeast's perspective, this is just multi-batch brewing, although I think that most breweries brew the batches on the same day. Realistically, I don't think there's a significant risk associated with storing wort for a day or two (assuming, as always, best practices sanitation). The risk isn't zero, though.
As others have pointed out, there are no costs associated with making starters using wort from a previous batch. IMHO, if you're buying fresh yeast for each batch ($3-8) it isn't worth worrying about the price of DME for a starter (<$1) anyway.