Start with whatever pretty-good mix you can afford or afford to make.
Then make it half as expensive by mixing it with an equal or greater amount of screened bark - inexpensive pine bark shreds, fines or small chunks (grit-sized, up to around BB size). Bark will hold a good bit of water if it is too fine (smaller than 2 mm).
Coarse bark makes fine mixes LESS water-retentive and BETTER AERATED if you add gritty-size shreds, like larger than 1/10 inch or 2.5 mm.
Pine bark breaks down slowly, like a year or more for fines, and several years for coarser chunks.
It won't cause anywhere near the nitrogen deficit that wood does in similar sizes, because bark breaks down slower and has some nitrogen itself.
You said "cheapest", so screen your own bark to the right sizes, starting with the cleanest bark mulch or other bark product you can find - pine, fir or balsam are best.
"Mulch" from my Home Depot is all soggy, smelly, dirty logyard trash. I only buy from Lowe's now. Lowe's had some pretty clean mulch, and VERY clean "fine pine bark NUGGETS".
Try to get bags that were never stored outside in the rain. Once they soak up water, they go into aerobic fermentation and produce toxins like organic acids and alcohols (bad for root hairs). And anaerobic microbes are NOT beneficial microbes in soil for gardening.
Screen it with hardware cloth, like 1/4" and 1/8". It can go faster if you use coarser screens to remove big stuff first. Then you can chop up the big stuff with a clean lawn mower or chipper, and re-screen that. Or use the coarse stuff as top-dress mulch.
I lay hardware cloth over some steel shelving I found, and lay that on top of wheelbarrows. Then push the bark around with the back of a rake, or prop the shelving on a slant and "pour" the bark down the slant.
I re-screen several times with different size mesh. I wish I had some 1/16 inch mesh to test! I read about someone who uses window screening (24 mesh!) to "de-dust" his bark fines.
If your store-bought or self-made "good" mix is finer than you need, screen the bark to be coarser. Try to get rid of dust and fine bark fibers. Keep sizes from 2mm to 4mm (ideally).
If the store-bought mix is already more than coarse enough (I never saw that happen), screen you bark to be a little finer.
If the store-bought mix is about coarse enough to have good drainage and very good aeration, screen the bark so you don't mess that up. Keep some fine stuff (1 mm) and plenty of gritty stuff (1/10 inch to 1/8 inch or 2-3 mm).
Perhaps test the bark you've just screened by filling a tall pot and soaking it well. Water should run out the bottom freely and leave the pot only somewhat heavier than it was dry - not HUGELY heavier with excessive retained water.
If the pot retains to much water, it has few or no air spaces left. Oxygen and CO2 can only diffuse in and out fast enough if they find some air-filled pores to diffuse through.
Gases diffuse literally 100,000 faster through a gas than through water!
A water-logged mix drowns roots and kills potted plants.
