Yes, there are multiple flights from Thailand to Zanzibar for under ฿ 29,996. The cheapest flight booked recently is on Emirates for just ฿ 27,087, but on average you can expect to pay ฿ 29,906.
There are no direct flights from Thailand to Zanzibar, but Cheapflights can provide you with all the information you need to find the fastest route with the fewest stops.
For as low as ฿ 47,755, Qatar Airways can have you in Zanzibar from Thailand in 15h 50m. Both multi-stop and direct flights can be found to your destination and back.
Flying on Emirates is currently your cheapest option to Zanzibar, with prices starting at ฿ 27,087. Another option worth looking into is Turkish Airlines, starting at ฿ 33,387 return.