IMHO, although fearful I'm in the minority, I don't think it has anything to do with the cost of the parts. "Enthusiasm" is an attitude. If you're gung-ho about your hobby, that makes you an enthusiast. In that context, the term could apply to any machine you build. If you spent weeks poring over parts, hunting down deals and saving to get just what you wanted, I'd say you probably came up with an "enthusiast" build, even if some rich drone bought an off-the-shelf system that is faster.
I believe that most "enthusiasts" enjoy being able to tweak their PCs. Usually then, this means they are capable of at least some amount of overclocking. If the builder has money, this likely means a Z-motherboard and an unlocked Intel "K" processor. On a tight budget, the G3258 can be overclocked even on cheap H81 boards. Somewhere in-between are AMD FX builds, which are often more tweakable than all but high-end Z boards (which may be able to set every voltage level of every part drawing power), but that may be slower at many tasks than even locked Intel i3 CPUs.