I just noticed why that slab was weird, it says XF40 instead of EF40(I think that's normal though). Anyways the truth to your question is about 20 % or less than someone else is willing to pay for it.
Anyways I recently purchased a 1830 rainbow toned half-dollar, for 60 $ in about VF20+ condition, that being said i think it might be worth 80-100+, but since yours is in better condition, I'd say don't spend more than 80-100 $. That's my opinion from a capped bust collector, but you know I could be wrong I haven't been in coins long enough to be considered a good coin collector. Hopefully someone with more experience can shoot you an answer.
so yeah, that was a little higher than I though, but I think if you could get it for under 140 your getting a good deal.
