In terms of welding, you can save a lot of money if you can do it yourself, in fact I suspect considerably more cars would be scrapped if we couldn't weld at home (or we'd buy a lot more fibreglass !). The labour involved in stripping down, cutting out, forming repair sections (not just whacking a plate over the rot) etc is very expensive at a professional level, whereas the raw materials are very cheap in comparison, so home repairs although time consuming are very cheap.
It's very similar to building work in that way, bricks are cheap but it takes a long time to put them up so the labour is the bulk of the cost.