It's not 10 but 20%. And it's always worth it. Money spent on learning schematic always returns itself.
I don't think its worth it. Level 27 mods are currently selling at cost of materials or below. The only profit to be made is off crit crafts. Avg cost for mod is around 1.2 mill which is below the cost of mats. Lets say you invest 6 mill to buy 5 and learn recipe. Now you're only making a profit off crit crafted mods. If you crit once out of 5 crafts then then comes out to a mere 200k profit per mod. You'd have to sell more than 30 mods to start turning a profit. I guess you could get lucky and learn recipe sooner but could always take +5 tries too.