I dont think you will find anything cheaper than dirt (garden soil). But dirt isnt recommended by most folks here (more chances of fungus diseases and doesnt drain quickly which can rot those baby rootlets)

What I am using for transplanting, which I have seen recommended several places is a 1/3, 1/3, 1/3 mix of Peat moss, Pearlite and Vermiculite. Its cheaper than buying prepackaged seed starting mixes, but it aint cheap as dirt. If you are potting up from peatpots then you dont need very big containers. 9-16 oz.(depending on the plant) plastic cups are the bargain way to go and they dont take huge amounts of mix.

What will you ultimately be planting them in? Though I haven't tried it personally, what about buying a bag of potting soil and mixing it 50/50 with the dirt from your garden?

Peat moss and garden soil are cheap. Perlite is reasonable. So I think a peat moss + perlite + garden soil (equal volume of each) should be good.

The reason I am suggesting some soil instead of completely soilless is because these are seedlings now, so they can take some nutrition.

mandolls suggestion is good. The Perlite & such can run a few $ though, so maybe not as much of that & add some pine bark fines (some cheapie small-particled pine mulches would do).

What a timely question! I was just wondering the same thing after I had already potted up my seedlings into garden compost. Probably should have researched first but I was so happy about my compost haul and figured hey, if they can't hack it in there, wait a second, holy carp, what have I done, I hope they don't get eaten by pests!!

I tried searching everywhere on the web and found no definitive answers, or even highly recommended solutions.

We'll see how it goes. My little seedlings are now fending for themselves in the great big world of compost and bugs and worms...wish them luck...lol =)

There's a reason that "pennywise, pound foolish" became an aphorism. You've invested money, time, and care in those seedlings; you want to be careful not to do anything to jeopardize them now. I'm not saying that you couldn't find a less expensive growing mix (doing it yourself will be cheaper down the road, but you may spend more at first buying each component in bulk) but I would be more inclined to look for a better price on a good growing mix than substitute a less expensive but possibly inferior product. IOW, I'd just try to be a better shopper.

I just returned from the landscape supply with a yard of nursery mix. The same thing the commercial nurseries use for their own potting. It is made of builders sand, fir bark and volcanic rock with a fertilizer added good for about 3 to 4 months in the pot. The cost per yard is $51 at their site. I have been using it for twenty years, and also re-use much of it a second time by adding back the fir bark that has been consumed. Landscape supplies are in most communities, mine is about 20 miles from me. Al

I live in a rural area so this may not be available for everyone, but did you know that many dairy farms "make" quick-compost from their cow manure? The stuff gets collected up and it's processed through these high-pressure steam, mixing, sludging, bacteria inducing, super sonic, blasting, turbo-charged flux capasitor machines. Ready-to-use compost comes out the other end and it's fabulous stuff. Already seasoned and airy and full of nutrition (but not so much nitrogen that it burns). The dairy here charges about $65 for a truck-bed load. I mix this stuff with peatmoss and vermiculite and "Bam" ... I have more mix than I have containers to fill. Depending on what I'm using it for depends on the ratio. I use a scooper -- 4 scoops Peat, 4 scoops Verm, 2 scoops compost for seeding. Make it 3 scoops compost for potting up, and 4 scoops compost for houseplants, outdoor containers, or raised beds. Vermiculite (course ground for gardeners) is pricey. About $60 for a 40 lb bag...but the bag goes surprisingly far and lasts several years.

Yeah, regular top soil (fill) can be problematic as it didn't provide my seedlings with enough nutrients. I ended up getting good results from 1/3 germinating mixture, 1/3 top soil (fill), and 1/3 humus/compost mix with a small amount of duck manure. All in all it wasn't too expensive (10$ for a large bag of germinating mix, 3$ for humus/manur, 2$ for 80 lbs of top soil). The mixture is a medium soil where the roots can spread easily. It also doesn't dry out too fast and provides the nutrients for the seedlings.