Scottg,
I do see your point. Priced at $110, the Lie-Nielsen (LN) butt mortise plane is a relative bargain when compared to other products in the LN line of tools. At roughly $300 to $400 for a LN #4 (depending on options like bronze versus iron, high angle frog, etc.) the butt mortise does seem inexpensive. There are certainly some very good quality used antique planes on the market for literally one tenth or less of the cost. The LN #4 is modeled after the Stanley Bedrock #604, which in good user condition, can literally cost anywhere between $25 and $150. Assuming the Bedrock is complete, undamaged, and not rusted beyond recognition, even at the high end of the price scale, it's still a better deal than the LN #4. Antique Bedrock #604 planes are still fairly common too.
Things start to get a little blurry when deciding to buy a less common antique user plane or a new equivalent LN plane. Some old speciality planes like the Rumbold are still reasonably priced, but they just aren't that available. The average user Rumbold is probably still going to cost about $60, and possibly a little more if it's in an online auction and two people really want it. You know how that goes! Sometimes just being able to buy a new quality tool from a current manufacturer makes sense when comparing it to the antique tool and its availability. I think for that reason, the available LN butt mortise plane is reasonably priced in comparison to the "sort of available" Rumbold. Based on your comment regarding the price of the LN, I think you agree.
There are also several factors that do make many of the LN planes attractive (to me at least.) Availability is obviously one factor. A perfect example is the Stanley #9 block plane. An original Stanley is cost prohibitive if you can find one in undamaged condition. Suddenly the LN #9, costing a few hundred dollars, doesn't seem so bad. Another thing I like about LN planes is their durability. Using the #9 again as an example, it's well known that original Stanley versions were prone to cracking. Using different materials and construction methods, the LNs are much more apt to withstand heavier use without incurring damage. The durability factor is particularly evident in LN models such as the #9, #10, #62, #140, and Sargent #507. The antique versions of those planes listed, even under normal use, were susceptible to cracking. I like the fact that LN has made available old patterns that went out of production decades ago. Some patterns that are just too hard to find, or are too fragile to use, are now available to everyone to experience, if he/she so desires. Having those specialty planes in its inventory costs LN money. Finally, and maybe most importantly, I really like that an entrepreneur started a company in the United States, and has managed to grow it, employ Americans, and stamp "Made In USA" on his products. Over the years, the LN product line has expanded and the quality has remained high. A testament to the LN quality and brand is really evident on the used tool market. Used LN planes seem to hold their value and often sell for near retail prices. LN planes are expensive, but in some instances they might be a better option, particularly regarding speciality planes. There are generally more economical choices to be had among common used antique tools, like a #4 bench plane for example. Still, I'm rooting for LN and consider myself an LN supporter.
Jim C.