Just chart it out. Look at the Benchmarks (Anand Bench is useful for this) for the tasks that matter to you, determine what price/performance measurement matters the most, and see if there's a detectable point.
For example, let's say Cinebench mattered to you:
So you could look at something like this and make an argument for any of the chips in question, but it depends on what is important to you.
G3258 - "Well, the cheapest parts give you the most performance for the dollar, so I'll get a G3258."
i3 - "Well, it's the fastest chip that's within my $140 CPU budget."
i5 - "Well, the i5 is the cheapest chip that will break 20,000 points, and that's the performance I need at minimum, so I'll get that."
i7 - "Well, the i7-4790 is the fastest chip that still offers >100 points per dollar, which is my line in the sand for bang-for-the-buck. The higher end chips are a much worse value."
You could do the same with SysMark or gaming FPS - however you wanted.
H81 chipsets and locked i5s make a lot more sense for people who don't bother with overclocking. (You know, the other 99% of the world.) Don't worry too much about them; but the flipside is, if you want to overclock, don't buy an H81 motherboard, like, ever. (Don't buy things that don't do what you will want them to, or which will only do them in very limited circumstances.)