I've done this a few times, on SNPA, SN Kellerweis, and Bells Amber yeast. I poured the bottle slurry from a SNPA into a 50 mL starter, to which was added 200 mL, then 500 mL, then 2L. This gave enough yeast slurry for the average ale (starters were stirred at 200 mL and beyond). It worked out great, as everybody else said just like 1056. It's more expensive to buy the beer than to buy the yeast ($8 vs. $6 here), but they don't give you a six pack when you buy a WL vial

You use about 275g of DME with this method which is about $2.43 if DME is $4/lb. By buying the tube you only need around 100g DME to get the same amount of yeast, $.88. Also count on at least a week to get the starter from the bottle fully grown up (1-5 days for the first step to ferment completely, depending on the age of the sample, then 1 day per step after).
That Kellerweis strain, BTW, is most excellent.