Why is Jet-A cheaper than 100LL? Shouldn't it be the other way around?
There are a number of factors that go into it:
Jet-A is used in a much higher volume than 100LL. All the jet engine commercial aircraft each day burn through orders of magnitude more Jet-A than piston aircraft burn through 100LL. That means that there is a much higher production volume, and therefore a cost savings as a result.
Buying in volume
It is pretty rare for a jet to taxi up and take on 50 gallons of Jet-A. Even the smaller jets rarely take on less than a couple hundred gallons. If you went up to your local FBO and said that you wanted 500 gallons of 100LL, they would find a way to get a discount for you.
100LL is special
Leaded fuels are not manufactured anymore for any other purpose than 100LL. There is a push in the aviation industry to do away with the leaded fuels altogether, but this isn't happening for a while. As a result, it takes a special process to add lead to the fuel, equipment not used for any other purpose. Jet-A on the other hand isn't incredibly different from diesel #1 or kerosene, at least in the way it is processed.
Because airlines buy so much fuel, they have negotiated contracts directly with fuel suppliers. These contracts help bring the price down because of competition, and the supplier is running a steady supply of fuel for the airline.
I'm not sure why you would think that 100LL would be cheaper, it's sold in low volume, it is expensive to make, and it is environmentally unfriendly and toxic.
Jet-A (and all turbine fuels) are far cheaper than 100LL (at least in bulk; retail prices vary due to local supply and demand as well as different regional tax regimes on aviation fuel) for several reasons.
Southwest Airlines alone burned through 1.9 billion gallons of Jet-A in 2015, dwarfing the mere 175 million gallons of 100LL that the entire US GA piston fleet consumed in the same year. This means that Jet-A is made and traded in large volumes, enough to merit refiners sometimes optimizing for its production over other products in certain refineries, while avgas is the bastard child of the refining world -- people would rather make other things that are easier to sell than 100LL.
Ease of Production
Jet-A, like all kerosene fraction products, is straightforward to produce: straight-run kerosene that has been run through a few sweetening (impurity removal) processes is mixed with mid-weight distillates from heavy cracking processes (catalytic crackers, hydrocrackers, cokers) and presumably processed further with hydrogen and clay to get rid of more impurities.
On the flip side, there are all of two TEL (tetra-ethyl lead, aka the "lead" in leaded gasoline) producers on this planet that I can find (Innospec in the UK and TDS Chem in China). Considering that the last few countries that use it in mogas are working on phasing it out, and very few road vehicles sold on the global market these days can even burn it to begin with due to being made to more stringent emissions specs, it'd be safe to say that the majority of TEL made on this planet is going into planes.
Atop that, avgas requires large quantities of high-octane blendstocks, but cannot use any oxygenates (they are prohibited by the avgas spec, aka ASTM D910, as GA fuel systems aren't compatible with 'em, especially ethanol). These high-octane blendstocks are primarily alkylate and reformate, both of which require specialized refinery equipment to produce. Furthermore, reformate is an inefficient source of high-octane blendstock (reformate is very heavy in aromatics such as benzene and toluene, which are better off getting distilled out of the stream to be used as a valuable chemical feedstock instead), and alkylation, while an efficient way to produce high-octane blendstocks, requires an acid supply nearby (either sulfuric or hydrofluoric), limiting where alkylation units can go. Finally, both of these blendstocks are in demand for motor gasoline production as well, which means that most refineries that produce them are filling mogas demand first and avgas demand second.
Jet-A is relatively easy to handle -- it's a close cousin to the kerosene on hardware-store shelves. It has a relatively high flashpoint (meaning you need to make a fine mist of it mechanically or heat it aggressively to ignite it), and this low volatility also limits worker exposure to the relatively limited chemical hazards it poses. Furthermore, it can be shipped readily via most refined-products pipelines without posing unusual hazards or risking excessive cross-contamination by or to other products.
On the other hand, tetra-ethyl lead is highly toxic, and 100LL is toxic not just because of TEL, but because of volatile hydrocarbon toxicity. While isoparaffins seem to be less problematic than normal paraffins from this standpoint, volatile aromatics (Benzene, Toluene, Ethylbenzene, and Xylene, or BTEX for short) are well known as troublemakers in the human body.
Atop this double handling hazard (volatility of the fuel and toxicity of TEL and light aromatics), pipelining 100LL is a much harder task due to the TEL contaminated transmix (contaminated stuff from the boundaries between products in a multi-product pipeline) it produces. Normally, transmix is reprocessed by refiners or dedicated transmix reprocessing plants to produce gasoline and diesel (with most of the kerosene fractions in the transmix being lumped in with the diesel for simplicity's sake), but the presence of TEL in transmix renders it unreprocessable as any more than 50mg/gal of lead in motor gasoline means it is contaminated and unsellable as per EPA regulations. Furthermore, shipping 100LL in a multiproduct pipeline risks contaminating other products with lead batch-wide due to dead legs and other such cross-contamination hazards, which means that in practice, 100LL is never shipped in multiproduct pipelines.
Instead, 100LL is typically railed or barged to terminals, and sometimes even trucked directly from the refinery as not all terminals have rail or barge access. This drives up shipping costs compared to every other fuel (save for ethanol and other fuel blendstocks that are surfactants).