Mary, you don't say how deep this box is.
Delivered by the truckload, I don't know what the cost of topsoil is. I mix a potting soil every year for perennial transplants. Since I have dahlia roots stored in peat moss each winter, that leftover peat moss accounts for 1/3 of the mix.
Another 1/3 is either my garden soil or bagged topsoil. The final 1/3 is my compost.
One needs to be careful about buying "topsoil." Often, it is a mix of wood chips and soil. Something we may want to be careful of is sand content, especially if the soil is coming by the yard. I don't want too much sand or wood and will snoop around the pallets of soil looking for a bag leaking some of its contents. Nearly always, there is at least one.
My compost isn't the best. Often, it includes garden soil to seal the pile in the fall. Even if I have it for over a year, I won't be turning it and it will likely be quite rough. I screen everything for the pots but, you know, at some point organic material will be dust. We aren't really stopping the decomposition by filling a pot or planter. Counting it as 1/3 is discretionary

but, it can be pretty good stuff.
If I was to substitute a commercial compost, I would want that to be a "composted" cow or chicken manure. A combination would be good. If the N-P-K is listed, it would likely be especially quality stuff. Something from a farm would be variable. The horse manure that I have gotten by the pickup loads has mostly been a mistake. In my experience, owners of horses make too much use of wood shavings, which is good for the horse and stable but not for garden soil.
Peat moss is dang near dust. Kind of a petrified organic material ...
Steve