Value? Well, that's a moving target, of course. My first bit ever was a model 1859 US Cavalry bit (similar to the one pictured, 2nd from the left). I think I paid $75 for it almost (cough, cough) 36 years ago. May have overpaid, but it belonged to my grandfather who sold it at his dispersal sale (24 head of Belgians and TONS of tack and equipment). And, should I ever sell my collection (doubtful) I'd almost certainly keep that ONE bit.
Anyway, they'll more typically fetch at least $150 (and easily more) these days. I've seen 'em go for $300+. There's nearly ALWAYS at least one m1859 for sale on ebay
Condition, as with most things, is VERY important and ya gotta be careful...there are a LOT of fake "US" bosses/medallions out there, even on genuine bits. Also, that m1859 bit itself has been reproduced for YEARS.
Easiest way to tell fake medallions...they are usually what I refer to as "muddy"...the letters aren't sharp, the edges of the boss itself aren't sharp and the background (behind the letters) is rough, not nicely etched. Easiest way to tell a fake m1859 bit...there'll be a "flare" where the mouthpiece meets the cheek, the port isn't a you bit looks more like a rounded off V and, more often than not, the fakes have stainless steel/shiny rein rings and slobber bars. One maker of reproduction m1859s is "MAST" and, if you see that mark on a bit, it's for sure a fake.
The original 1859s came in 3 port heights, along with the ring bit (nasty, nasty thing!) pictured at the far left. That ring (also found in lots of Mexican bits and some European ones as well) went AROUND THE LOWER JAW if you can imagine that! Fortunately very few of these ring bits were issued by the gov't so fewer horses had to suffer. 'course that also makes them very hard to find and pretty high priced. Tough to find these, assuming the seller knows what they have, for less than $300 and they can bring LOTS more.
Collectors sometimes do refer to the m1859 as a "#1", "#2", "#3" or "#4" which is a reference to the port height (respectively 2.25" for the ring bit, 2.0 inches, 1.5" and 0.5 inches).
The other 4 bits in the picture are US (federal) artillery bits from the Civil War. Note the slots (versus rings) for the reins...keeps the lines straight...and the upper rings (except that one 2nd from the right which I've since replace with one which HAS rings). The first of the four is a m1859 first pattern bit...it has plain, brass-faced bosses. The next one is the m1859 SECOND pattern bit. It's built a little heavier than the first pattern and has "US" bosses. Next is the m1863 artillery bit. Note the "slot" versus hole for the curb chain (or curb strap) and the sharper "S" curve in the shanks. This model has a beautifully designed "intertwined" "USA" boss. Again, this bit was issued with the upper rein rings but this particular example (and quite a few I've seen) have had the rings cut off. The last bit (far right in the picture) is a modification of the bit to its left. A post-war order stated all new bits and any bits being repaired should be tinned (instead of brass plated). At the same time the heavier bosses (like those found on the m1859 cavalry bit) replaced the lead-filled, thin brass shelled "USA" boss.
Value on these artillery bits runs somewhat higher. I'm guessing they were produced in much smaller quantities. I'm not really sure what they're fetching these days (I've not been paying much attention lately) but I'm guessing they might start @ close to $200 for good ones.