I understand where you're coming from in terms of bhp/£ in some sense, but you need to understand the characteristics of how engines behave in sense.
You may only get 50bhp for 3k spent at mountune, but having been out in one i think personally you would be lucky to do better for less on the duratec engine in the fiesta, especially when considering that it is a ford manufactures approved modification is VERY hard to do.
Alot of the money goes into the labour that it takes to design, manufacture and building the parts for the cars, hence why you might think its expensive, when you actually break the costs down to each bit you can see where your money has gone into making it right.
You can always buy cheap by all means, but you'll be buying twice when it turns out its some crappy chinese made bit rather than something thats been done from scratch here in blight
Turbo engines obviously will get greater performance gains for less money, thats always the way with forced induction, but some people prefer an N/A engine just because of how it acts and reacts.
Plenty of people have s/c'd or turbo'd an ST, but you do hear of alot of them having reliability issues further down the line just because the engine with standard internals is simply on the edge of what its capable of handling. Things can get very expensive very quickly if you're chasing big power!
Thats my views on it anyway, i love my engine for what it does and thats why people buy them and keep them that way
Hope that helps