I feel that when you do something that you should do it right. When that theory is brought over to my planted tank, specifically my substrate, I would rather have a plant specific substrate to provide nutrients to my plants so that they can get a good start. I do not want to have the hassle of using an inert substrate (ie. sand) and making root tabs on a weekly or biweekly basis. I simply feel the the money is well spent from the beginning because it gets you off on the right foot and you dont have to be constantly playing catch up.
I do understand that plant specific substrates have their "expiration date" where they run out of nutrients, but I do feel that since you are dosing your water column with a form of EI dosing that the water and nutrients do make it down into your substrate and keep it nutrient rich. With sand you need to start with an inert substrate and get it to the point where it becomes nutrient rich, and IMHO, that is too much of a hassle.
When I need to value things in life, I value my time at $20/hr. If i would need to spend 15 minutes once every two weeks to purchase materials, make, insert, and do other maintenance (such as stirring) to my sand substrate it would cost around $130. At this rate I could have gone out and purchased 3 bags of eco-complete for 22/bag, a years worth of ferts, and some new co2 accessories and still spend less than $130.
-Chris