One can get a good sense of relative costs in the 1770s just by comparing the enslaved man, valued at £225, with other items on this very inventory: a tea table cost £10, a kitchen dresser (a hoosier?) cost nearly £3, a couple of rugs cost £2 total, a silver cream jug cost £3, 6 silver spoons cost £7, a half-dozen chairs cost £6. John Henry sold rifles in this period for about £7 or £8.
Indeed, everything else that John Henry possessed, his household furnishings and all the tools and stock from his trade, were worth about (a bit more, but about) what this enslaved man was valued at.
So at £225, this enslaved man had an extraordinarily high value.