The main issue is scarcity and rarity in today's used/secondary firearms market, although high quality of manufacture is a secondary related issue. Quite frankly, although the American Mosins were a drug on the market in the 1920's and 1930's, finding one today in decent condition that does not bear "The Taint of Bubba" at $149 may qualify as a minor miracle.
Originally, the Russians out-sourced Mosin-Nagant rifle production to American manufacturers like Westinghouse and Remington in order to get their armies equipped for WWI. As part of the process, they sent Russian Army officers to the plants(Remington actually built housing for them on-site, IIRC) to act as QC inspectors - and since any defects found were likely to subject the manufacturers to penalty clauses, the inspectors were very motivated to find defects in order to save the cash-strapped Tsarist regime money, and the manufacturers were similarly highly motivated to eliminate defects so they could stay in business.
However, the Communist Revolution of 1917 put the kibosh on the whole deal, and Uncle Sam wound up bailing the manufacturers out by buying up the un-delivered rifles. Since they were not chambered for our standard military .30-06 rifle round, most were stockpiled and some wound up being used as drill/training rifles for new recruits as the US Army geared-up for WWI.
Thousands of these American Mosins wound up being sent to Russia anyway, but in the hands of American doughboys as part of the little-known, inconclusive "Archangel Expedition" wherein the US tried to intervene on behalf of the White Russian forces and secure the international port of Archangel against the revolutionary conflict. The thinking was that the Archangel Expedition might as well use the Russian rifles, and scavenge Russian 7.62X54 ammo as needed locally, thus freeing up .30-06 Springfields and Enfields for the AEF in France. At the end of the Archangel Expedition, the doughboys simply abandoned their Mosin rifles in Russia and sailed home.
So many American Mosins were left over, though, that Uncle Sam finally wound up surplus-selling them to large sporting-goods wholesalers and through the NRA. Used to be that one could mail-order an American Mosin for $20(IIRC) - with a case of ammo thrown in, just so the rifles would move out the warehouse door. However, during the Great Depression, money was tight, and the rifles STILL didn't sell.
A major sporting goods store in New York, Bannerman's, took a lot of their Mosin rifles in inventory and re-chambered them to .30-06, hoping that this would help the rifles to sell(much like Gibbs Rifle Co. butchering full-size Lee-Enfield rifles into
ersatz "Jungle Carbines" and "Tanker Rifles"). The Bannerman conversions were apparently done properly, where the barrels were removed, shortened at the chamber end, then re-chambered in .30-06 and reinstalled/headspaced. Unfortunately, a lot of "shade-tree gunsmiths," gun-butchers and Bubbas simply ran a chambering reamer into the original chamber without shortening it, resulting in a grossly oversized chamber and a rifle that explosively disassembled itself upon firing. Since it's hard to tell a safe conversion from an unsafe hack-job without taking a casting of the chamber, the conventional wisdom is to avoid converted Mosins like the plague.
Considering all of these factors, a dealer who is selling a good-original-condition American Mosin for not much more than a crudely-manufactured Soviet-era 91/30(and less than half the market price of a Finnish Mosin-Nagant) either doesn't know the value to Mosin collectors, or figures he can't move the rifle to the uneducated gun-buying masses that darken his doorstep.
Bottom line - BUY IT NOW. Then check out
www.gunboards.com to see what the cool stamps and markings on the rifle actually mean - you might find even more neat facts about the rifle's history, like maybe it was captured from the Russians by Germans, Austrians, or Finns, and used against its own country of origin...