Sigmas are just as efficient as the Lambdas that use the same driver design: SR-Lambda and SR-Lambda Signature. You can measure this with your cell phone using a dB level app in a pinch. They do however require a bit more gain to get the same volume at the ear position since there's a greater distance between the ear and driver in the Sigmas than the Lambdas.
The only time I've seen some of the low prices you're advising has been on auctions that nobody caught or amps with damage. Real world value of the SRM-1 mk2 has been about $400-450 and the T1s go for between $450-650. Pretty much been that way for half a decade at least.
Indeed on the Sigma's requiring more gain on volume to match a normal Lambda due to the driver distance to the ears.
With the prices, I don't know what the average price of the SR1 MK2's over the past half decade is, but the prices you mentioned is not the average standard prices for Stax gear that I've been monitoring eBay attentively a little after I joined here, almost the past 3 years.
Just to put things into perspective, a little more than just 2 auctions Lambda Pro + SRM1 MK2 (Pro and Normal) sold for around $350-500 on the Aussie ebay domain back in 2011, at that time I would've snagged up on the deal, but I didn't have enough funds so I landed myself the SRD-7 SB MK2 + LP in original boxes good condition for little less than that amount.
Koss ESP950 packages come and go at around $400-500 here. I've seen two Sigma buy it now auctions back then here that went for $400 and 500 respectively. I purchased my Sigma Pro's with original box for $500 buy it now. A little over 1.5 months ago, a Lambda Signature + SRM1 MK2 (pro and normal) was sold for final auction price of $600 posted with original box and original pads. A SRM-T1 went for $331 USD from a seller in Vietnam little over 2 months ago, not a scam since the seller posted pictures with his username that I requested and he had over 500 feedback.
I've also seen crazy buy it now's on the UK ebay domain, SRM1 MK2 PP models sold for less than $400 but no outside international shipping. The US ebay domain is a hit and miss with good Stax deals, usually the amp auction prices go really up high when there is attention in the market for them (or a particular model) but sometimes people just snag up on cheap buy it now auctions fairly fast. I've rarely seen good deals on the German ebay, rarely, everything is usually sold very very high (not surprising since they have customs and import tax etc). But I was very lucky to have my friend obtain an SR-Omega when he was there for $3400-3500 AUD after Euro conversion when the AUD was worth more than the US at the start of this year.
Don't even get me started on the prices that Stax gear go for on Yahoo Japan auctions, SRM-T1's go for around 35000-40000 yen, I've even seen crazy package auctions of old Lambda Pro and Lambda Nova Classic + T1S deals for 55000-60000 yen. Heck if you check now there is a T1S or W at a buy it now for 37800 yen, common prices for T1 amp's, even after a proxy service such as Kuboten, Rinkya, buyee etc, you'd still get the amp's within $500 shipped etc.
Then there is another issue of demand and supply, unobtaniums are only going to get rarer and harder to find at appropriate prices, I'm not sure if you're observant with the Stax gear prices sold here on Headfi F/S section, but before I registered going back to my lurking days from 2004 before the forum system change and before I even gave a crap about Stax stuff and when nobody really gave a schit about Stax gear, Lambda Pro's selling for as low as $150 in working order, nothing broken, SRM1 MK2 (both normal bias) going for around $200-300 shipped, this was around the time where everyone here was raving on about the HD600/650's, before the AKG K701 came out. There are still some in the archive subforum section.
The Euro ebay domain is more expensive than anywhere else. Sigma's being sold at $3000, T1 + Lambda Pro being at $1600. If you want good deals, look elsewhere. Just my observations.
Edited by DefQon - 7/30/13 at 11:51pm