Fondant is very expensive to buy. I have stopped buying fondant and make my own. I do cakes and cookies occasionally out of my house, so I have more time than money.
I use Rhonda's ultimate MMF (http://www.cakecentral.com/cake_recipe-3183-5-Rhondas-Ultimate-MMF.html
) and personally I find it very cost effective. About $3 for a batch that yields about 3 lbs. (sometimes more as more humidity usually means more powdered sugar.) I find the cost to be comparable to RI (the kind of RI I make uses meringue powder.) The powdered sugar cost is similar for equal amounts of RI and MMF. The cost of a bag of marshmallows isn't much when comparing to the cost of MP I use. I find the time it saves in frosting is worth the very slight price difference.
I'm sure it's personal preference as well. My husband and I actually did a taste taste of butter cookies with RI (the recipe given in the how to decorate cookies tutorial) and Rhonda's MMF and we both preferred the MMF. Hope this helps.