It depends somewhat on what your "top soil" really is. Top soil could be anything from a sandy loam to a sticky clay. Frankly, I would NOT risk it.

Vermiculite is a very nice additive to your soil. And it doesn't break down, which means that unless it washes away, it's going to be there helping your garden for years to come. The success of your garden will add to your pleasure of the experience, and your likelihood of continuing to garden. So why skimp now?

Also, there are different "grades" of vermiculite. For a raised bed, I would stick to the coarser stuff. If you find bigger bags of coarse vermiculite, it might lower your cost a bit.

My father built raised beds, and without asking me, he had someone deliver "topsoil" and he filled those beds with that. What he got was very bad, compacted clay. The stuff will NEVER break down, and nothing grows well in the compacted container that he has created. His only option is now to have a significant part of it removed in order to put in something more suitable for plant growth.

You don't want to be in a messy situation like that. Think of what a hassle it'll be later to have to remove and set aside the top 6" of good stuff so that you can scoop up the "topsoil" and then refill the whole thing.

Best to do it right from the beginning and save yourself the headaches later.

Purchased "top soil" is very different from compost.

I would just fill the bottom 6" with your mix now and leave room for adding additional bulk compost in the future to slowly fill the beds.

If I remember correctly the book recommends filling the bottom with sand in that situation. If sand is okay I'd imagine "topsoil" would be ok too.

I've actually read posts where people fill the bottom with things like sealed empty plastic bottles. Anything to take up space.

When I needed to fill a deeper bed, I put leaves, sand, straw, bark mulch in the bottom and used the recipe on the top. I knew that eventually the stuff on the bottom would decompose. I had a slope, so needed to fill one end more to get the boxes level. Seems to have worked pretty well. As long as the material will become compost, I think whatever you can get your hands on would be better than topsoil. Also, many garden stores and big box stores will sell broken bags half price.
Keski

There will be some who take issue with what I will add here. It doesn't matter because it's true and they are not my words, but Mel Bartholomews words: "Mels mix may be the most costly part of SFG, yet at the same time it is the most cost effective. How can it be both? The answer: if you try to skimp on this item, you'll be disappointed in all the rest. But, if you do it right, all the other advantages of SFG will fall into place and you will be the richest gardener on the block. Sorry to be so adamant, but this is really what makes SFG so different and successful. We have never had a failure of SFG except when someone decides to skimp on the ingredients to save a few bucks."(pg.87 in the All New SFG book). Go back and read many of the issues in this forum and see how many can be traced back to the soil. Just my two cents worth

Snibb is correct. It doesn't pay to cheat on the soil ingredients. Where my slope made the boxes 12" deep, I used compostable materials to fill in the bottom of the deep end. Since Mel has you add compost as you harvest, I figured composting stuff would work. Any way, I did harvest some awesome parsnips over the weekend from that spot.
Keski