>> I would love to find something to put in the bottom half to help with the expense of soil.
Bark nuggets are MUCH cheaper than potting soil. They will provide excellent aeration and drainage. If the only nuggets you find are larger than 1/4" to 1/2", you might want to use bark shreds as well, to "fill in the cracks", so potting soil doesn't sift into the bark layer.
You can make bark shreds by screening CLEAN bark mulch, or by breaking up bark nuggets (they are usually cleaner than "mulch") and then screening them. A chipper/shredder or lawn mower will shred bark much more easily than it breaks up twigs.
If you go a little farther, and use lots of bark shreds and bark powder, the bark layer will become almost as dense as the potting soil, and you'll lose some of the great drainage and aeration.
However, you will also have created a very inexpensive potting-soil-substitute. You might consider mixing screened bark (especially pine, fir or balsam bark) with your current potting soil to reduce its cost and improve its aeration and drainage. It's easy to find bark coarser than coarse Perlite, and it is cheaper than Perlite. It will not be "sterile" as Perlite and very good potting mixes are, unless you are rich enough to buy "orchid bark".
If you improve the drainage of potting soil a lot, you probably will also reduce its water-holding capacity. That might mean more frequent watering, but it might also mean eliminating root rot due to water-logging.
