it depends on the vegetable. for onion and lettuce, 8 inches root depth is fine. beets and toms go four feet. but you do not have to have topsoil all the way down.

How much topsoil do you have now, and what type is it (sand, silt, clay, high or low organic matter)? What is the subsoil like?

Many gardeners have started with just a few inches. I made my first garden in a place with about 4" of clay topsoil over heavy clay subsoil. You can amend it over time to improve the texture. The point is that you may not need to buy 8" of topsoil for a garden. Or for a lawn, for that matter.

Cool, another Saskatoon gardener!!!

If OP is in one of the new developments, I'm guessing some of the topsoil has been excavated out.

Almost my entire "no dig" garden started in rectangles, one at a time, of 2X4 boards, so much stuff has been grown in 3 and a half inches of compost on top of sand that goes down, as far as I know, anywhere from several feet to the iron core.

The beds' "soil" be deeper now, but that just happens over the years.

Topsoil is the top 4 to 6 inches of soil. That is where the majority of organic matter would be and where the majority of the aerobic bacteria would be active.
I have heard from "landscapers" that will tell you that lawns, turfgrass, needs anywhere from 4 to 12 inches of "topsoil" apparently determined by how much they think they can sell you.
Rather than simply accepting what this person is trying to sell you look at it and specify that you want soil that is about 92 percent mineral and about 8 percent organic matter. "Topsoil" can be anything the seller wants it to be since there is no real definition of what "topsoil" is.