Compost is really what one makes in a bin/heap from a mix of green sappy stuff and brown dead stuff (high Nitrogen and high Carbon containing materials)
Basically it's what you'd find if you scraped away the top few inches in a woodland, with decayed weeds, leaves and a healthy ecosystem of fungi, bacteria and creatures.
The stuff labelled as "Compost" in garden centres is actually "Growing medium"
it contains compost, but also top soil (which will also contain compost naturally) and sand maybe and other added trace elements and fertilisers ( hence the element balance of N:P:K you see on the bags), all free from weed seeds.
Most peat free mixes have a high level of composted wood chips in them, and often the cheaper ones are quite coarse, the wood isn't composted for long enough ( in my opinion) and with lots of bits of wood visible .
You can grow plants straight into Compost, but it runs out of nutrients quite quickly, and often doesn't have the structure needed to support a growing plant physically.
Also you'd need to produce a heck of a lot of your own compost, and unless you were a real expert, your compost would still have weed seeds in it
Pay as much as you can for Growing medium!