I don't fly thru that airport, and if I was flying onto the DR, for a visit, I would not bother with any booze at duty-free. The DR has excellent rum. Like the previous thread, and while it was about DR rum in a DR duty-free, the same concept applies. If it's not a substantial amount cheaper, it's not worth the effort to find it if it's even available, stand in line and haul it home.
Define "a lot" cheaper please. Your first example is about $5USD cheaper than at home, and it's not worth stopping for regardless. The 18-year-old...if it was available I might stop for, but it would have to be an especially good price since again I can order it and have it delivered...but that's just me.
I checked in both STI and SDQ duty-free and they don't currently carry Flor de Caña, so I could not get it to take back home either, even if desired.
For me, it's more about being worth the effort than just the potential savings, and if I'm not saving say $50 I'd rather sit at the bar in the airport enjoying an overpriced beverage or 2 than shopping for things I can order online and have delivered to my home.