some of those places are still pretty expensive although their stuff is better or least graded better. exurban areas near where you live are likely to have reclaimed lumber, which is dirt cheap and may have added character. steel just got a lot cheaper - $10 for 20ft of 1/8 wall 1/2 x 1" cold roll welded tube. $6 for 1/2 x 1/2. building supply recyclers near where i live still have solid core doors piled up.

i think the appeal of Ikea as stock is that its already cut down and comes with fasteners/brackets/etc. but you can come out ahead and have considerably more design flexibility if you know how to shop.

i built maybe 60' in shelving for my bedroom. light steel frame, inset reclaimed pine flooring. the whole thing was probably $50 in materials. and when i leave it on the street when i move out, someone is likely going to want to take it its not going to look like an old cardboard box.