Air-cured Burley typically has a pH of 5.85. Flue-cured Virginias average 5.4. Remember, though, that pH is a logarithmic value, so a pH of 6 is 10x more alkaline than a pH of 5. Unfortunately, I don't know the average pH of fire-cured tobaccos.
There are other factors in nicotine level, of course. Some varieties are naturally higher in overall nicotine to start with. The upper leaves are higher in nicotine than the lower leaves. The manner in which the initial yellowing step is done affects the conversion of starches to sugars. Ribs have tons of nicotine compared to the leafy parts. The temperature of the drying step has an impact. I'm not sure how fire-curing affects the pH, but my understanding is that it does somehow contribute to an even higher level of free nicotine.
On the topic of added sugars, the type of sugar determines whether it is considered a reducing sugar or not. Sucrose is non-reducing, while fructose, glucose and invert sugars are reducing. So maple syrup, which is primarily sucrose, doesn't impact free nicotine as much as some other casing sugars.
BTW, this is just info I've gleaned from various industry and government articles. I'm no tobacco expert. The majority of the info comes from this outstanding article, which had been shared on this forum in the past.
