Leastbest -
It looks like a 20s or 30s Style 0 Soprano, but I can't quite see the side. It looks like there might be binding, and if there's rosewood binding between the sides and the back and top, you've got a 20's Style 1; if the binding is tortoise shell, you've got an early to mid 30's Style 1. Could also be a late 40's Style 1, as they didn't use the headstock decal for a few years in there. A close up of the side of the fretboard will show the type of fret, and that will tell us.
As to why the high numbers cost more. Bling was already mentioned. Rarity was mentioned earlier, and that's certainly a big factor, too. They made a lot of Style 0s and there are still plenty of them around - I got my early 60s 0 for $150 bucks in the early 90s - now they can be had for $400-600. Still not too expensive and still common.
They didn't make as many 1 and 2s, and they made a lot less 3 and 5s. They were the only manufacturer on the mainland making Taropatches, and they supposedly introduced the Concert - those are both VERY rare and cost quite a bit. Two Taropatches were sold in the last six months that both went for $2.5K+. The older tenors and baritones are also rarer, and correspondingly more expensive.
Then there are the koa ukes, which are definitely rarer, and so, they are the most sought after and expensive. $5K for a Style 5K is - appropriately - about what they cost. But - a style 0K, which they made, which fetch north of $1K easily and more than $1.5K is not unheard of. They just didn't make many at all.
And then, there are Martins made for other manufacturers, which are unique styles. The 3K version they made for Wurlitzer in the mid 20s is outstanding looking, and I've only ever seen one of them, but they probably made a few. The ukes they made for Oliver Ditson in the 20s are also very pricy, even the style 0 - which are rarer than even the 3 and 5 vintage ukes from the same period. Those with a dreadnought body are more common, those with a plan Martin style body are rare as hens teeth. Retrofret has one Ditson Style 2 going for $1.6K
http://www.retrofret.com/products.asp?ProductID=3608&CartID=44885311292011
So if you go for the common style 0, you've got the best value. As Rick and others note, the workmanship is as good as on the top-line ukes. But, rarity, style and better materials DO impact value immensely.