I have a 300 sf raised bed and purchased topsoil and amended heavily. Not a problem.

Dan

@Dan: What did you use to amend the soil. I plan to do the same as jamesgindc.

James, I think you can do Mel's Mix for close to the same price as you are calculating for topsoil. My mom just did 76sf 8" deep for $155.

I used several different purchased composts, compost from the pile, etc. Your manufactured soil mix should be good in and of itself for the basics and simply adding OM should do it. You'll get settling and have to refresh, and compost does that well.

Dan

I dont use Mels mix either, my soil is 1/3 topsoil, 1/3 coarse sand, 1/3 peat moss. I tried using straight topsoil one season but found it compacted and was to hard to work by hand so I switched,, best thing I could have done for my 168 sq ft of beds, I also modified the width of the beds to 40in wide, just takes 3 8ft boards to get a 3'x8' planting area.

In order to keep costs down, I discovered that Lowe's and my local Agway will sell torn bags of topsoil, potting soil, manure, peat moss,etc. for half price or less. I had to fill a sloped bed that was 12" deep on one end. I was able to get 4 bags amended topsoil, 2.2 cu ft. peat moss, large bag of MG garden soil,large bag MG potting soil, 1 cu. ft. potting soil, 1 bag manure all for $18. I also threw in sand, leaves to fill the deep side. Then I used Mel's Mix in the top six inches. The additional materials for the mix cost about $57.
Keski

I used purchased "topsoil" which was really mostly compost. Then I added peat moss my own compost and municapal compost. Spent about $40 on soil for 49 sq feet. If I were to do it again I would use my compost my leaf mold municipal compost and peat moss and probally only have to spend $16 for the same sq footage.