I may give Propper starter a go for my next beer.
Proper has gone down somewhat in price the past year or so. It used to be $6 for a 16 oz can, now it's $3.75, ($15 for a 4-pack, MoreBeer).
DME has gone up quite a bit lately, it's as high as $6 a pound now if bought in 3 pound bags (MoreBeer). My LHBS sells it for $5.33 a pound ($16 for a 3# bag), while it used to be $4 a pound.
Now 1 pound of DME will make 4.5 liter of 1.037 starter wort, or $1.33 a liter (MoreBeer pricing). Using
Proper will run you $3.75 a liter,
a difference of $2.42 a liter,* almost 3x the cost compared to using DME, just for the convenience.
Not a big issue when you brew once in a while, even once or twice a month, but if you brew more often, or propagate yeast for keeping or sharing, or brew larger batches, it adds up.
* Mind, one liter is not quite the amount needed for most pitches, 1.6-2 liter starters are more common for 5 gallon batches of ale. That allows one to ranch some too. So the price difference between starters made with Proper and DME becomes more significant, and double that, in case of Lagers.