I'm offering 4 alexandrite per 1 high-purity bayld. Feel free to contact me here/messages/in game if you are interested.
I based this exchange rate on the 30k alex needed and (as I understand it) over 10k HP bayld needed, plus considering the gil value of both. Remember that you can exchange 10k regular bayld for 1 HP bayld.
edit: the NPC to exchange bayld for HP bayld is Runje Desaali in Eastern Adoulin (by the Adoulin castle gate).
edit edit: going up to 4 alex per HP bayld