I'm not sure what's cheaper, honestly, buying the powder to make your own with the other ingredients in the recipe or buying the tubes of paint to do it yourself. Either way, it's cheaper than getting henna done typically. The best henna is expensive. Gorgeous, but expensive.
The auction is just the recipe. And typically they last about a week. However, if while they are drying, you apply lemon or lime juice, they last a bit longer than that. The only other tool I would suggest is something like a toothpick to altar the designs or add details like slashes in the paint. For example if you apply large dots of the paint and want to make them more like leaves, you would use the pick to divide them and pull the paint to a tip on the dot. The other thing is, if you do it on any skin with hair, even small ones like on the hands, while it dries and leaves the design on your skin, it's going to hurt a little if you pick at it or as it pulls a bit on those hairs. When it dries and flakes off it leaves the design on your skin. The paints usually dry fairly quickly. The tubes of paint, I know typically dry within the hour. The homemade ones typically don't take long either.
This actually looks like harkkous..or what I know to be harkkous. Tunisian black ink henna that goes on top of the red dye henna? I have quite a bit of it, but mine is from my boyfriend in tunisia. The stuff I have from him, lasts 2-3 weeks, if not longer. Depending on your skin and how well the color absorbs.
I only know so much about it, so you very well may be right, but I'm not sure. I got henna for the first time a few years back and just really liked it after that. I know a handful of people that do it and I know some about the designs but I don't know everything about it. Thanks for the info :)