Maybe not considered 100% hydro but definitely not soil is coco and blumats. It's basically a gravity fed drip type system with no runoff so no wasted nutrients. I reuse the coco more than once as well. I can fill five 3 gallon fabric pots with one 5kg bag of coco for $17. That's $3.40 a pot and since I typically use the coco twice that drops down to $1.70 a pot for the grow medium. For nutrients I use inexpensive VitaGrow which is just calcium nitrate, MKP, a micronutient blend and dirt cheap. It costs me less than $10 in nutrients for an entire grow start to finish. So $2.00 per plant for nutrients and $1.70 per plant for coco and the cost is under $4.00 per plant for a grow. That's going to be much less than both hydro and soil growers growing indoors under lights. I didn't include the costs for electricity, lights, pots, misc, etc... because those costs are the same as soil and hydro growers growing indoors under lights in a tent or grow room. I also do take advantage of outdoor light and put the plants outside weather permitting while in veg before they go into the flower tent and I setup the blumats.
There is debate as to whether coco is actually hydro but in
my opinion it's a hybrid and if done properly is less expensive than both hydro or soil unless you're growing outside in the ground in natural soil under the sun. Which some people do and get excellent results. But that depends on the native soil they're growing in.
Coco + blumats is probably one of the most inexpensive and easiest ways to grow. I've done multiple types of hydro and have grown in soil. Once I started growing in 100% coco with blumats I never looked back. This method of growing goes against most of what people are told you need to do when growing in coco. For instance, no runoff. People think you need to water to runoff daily with coco or else you'll get salt buildup. You don't. Feed at a low EC and you're fine. The nutrient companies have people overfeeding their plants and that's what causes salt buildups. They say you have to use CalMag. You don't and I never do. In fact I have never purchased a CalMag product in my life. There is plenty of both in VitaGrow. There should be enough calcium and magnesium in any nutrient line to begin with. The fact that it isn't shows how the nutrient companies are purposely leaving it out so they can sell another bottle of product. But that's an entire topic on it's own.
Anyway, since this is more a less hydro vs soil thread I just thought I would throw out a hybrid type of growing that I think many people might be interested in looking into.
3 gallon fabric pots, 100% coco coir, VitaGrow nutrients, blumat watering system, 600 watt HPS, 4 x 4 tent, 4 - 5 weeks into flower (guestimate). No runoff or CalMag for the entire grow.