Are fusors a cost-effective source of helium? It's not hard to imagine geology that didn't give us convenient concentrations of terrestrial helium, or that someday soon we'll have exhausted those natural sources.
How cost-effective is existing fusor technology at producing helium?  I'm just looking at a ballpark estimate to produce, say, 1 standard cubic foot of helium from other elements.  (Note that presently the market rate for that amount of helium from natural sources is about $.10.)
Update: Since the orders of magnitude are so far off that any net-negative fusor appears to be non-economical, how much usable 3/4helium would (contemplated, efficient) fusion energy reactors producing enough electricity for the U.S. (say, 1TW constant output) produce?
 A: Transmuting chemically significant quantities of one element to another using nuclear reactions is not cost effective for any naturally occurring element.
Nuclear physics is the end of alchemy.
Two examples I happen have off the top of my head: the "Fat Man" and "Little Boy" nuclear weapons deployed in the second world war each involved about $10^{24}$ fissions, and therefore  produced 1–3 grams of free neutrons (depending on whether you count the neutrons that were reabsorbed in the chain reaction). That's roughly the same number of useful neutrons that will be produced at the Spallation Neutron Source (SNS) over its expected lifetime of 30 years.
Wikipedia reports that the highest neutron rate achieved by a fusor is about $10^{11}$ neutrons per second, a factor of a million smaller than the SNS. Alpha production rates would be comparable. That's roughly a microgram of helium per fusor per decade.
To address your update:
the reaction
$$
\rm {}^2H + {}^3H \to {}^4He + n
$$
releases about 17 MeV.  A gigawatt power plant would produce such reactions at a rate of about $10^{20}$ per second, or about two moles of alphas and neutrons per hour.  I picked a gigawatt because that's a typical size for a large commercial reactor, but this rate would be for a reactor in a fairyland where all of the reaction energy went into power production.  This is also the scale of the ITER reactor, but that machine will operate in pulses of less than 1000 seconds each.
