Hmmm, not TOO much! partly as it will make the compost heavier and less free draining.
Back in the eighties of the last century (1980s that is!) I used mushroom compost for potting, with a little woodash and seaweed meal added. It grew good plants, the only snag for replicating that now is that most mushroom compost I see, and some I have bought, is less rotted with bits of straw. Would be good if dark and well decomposed, it is light and has no weed seeds.
Then last year I had success with cow manure as potting compost, well rotted, eighteen months old or so. Also my own compost when a year old and put through a coarse sieve. Should have added sharp sand to imporve its drainage but it was ok!
A few ideas to reduce your expenditure.