I reckon you have to define "economical". Of course the cheapest way to link ammo is just with your hands, and once you get things arranged and get calibrated on the force required, you can do it fairly quickly. It also give you a chance to spot "loose" or "tight" links -- but of course it gets tiring, and it's not too hard to under- or over-insert a round by enough to mess up your feed mechanism.
You've already found several sources for linkers I'm sure, but here are a couple I've found:
Link Loading Machines, Literature, and Accessories for Belt Fed Machine Guns Belt Fed Plus - Accessories, Parts and Manuals for Belt Fed Guns
Ammunition Linkers and Belt Loaders
Most people wouldn't call a $300+ tool "economical", but it is a one-time expense, so you have to decide whether you can stomach that.
So far, the only linker I own and have used is one for the 9mm links feeding a Freedom Ordnance FM-9 upper. Of course those are different than what you're loading, but the basic mechanism and technique is the same. I would not say using a linker is faster than loading by hand, but it does require less effort, and you're less likely to over- or under-insert. Also, instead of picking a link, picking a round, assembling them, and inserting, you're handling a handful of links at a time, then a handful of rounds, then inserting them all at once, so I think that task is a little better organized than doing each task one at a time in sequence.
Also, if you haven't figured it out yet, one linker will not do both M13 / 7.62 and M27 / 5.56 belts since the spacing between rounds is different. If you are going to load both those rounds, what you get is a linking press with interchangeable plates for the different rounds. It will cost you more for one type of round, but will be about the same or a little less by the time you're set up for a second (or third!) type.
