Talk to Runje Desaali (J-10) in Eastern Adoulin near the castle (around the corner next to the leafkin).
BG
After a short discussion, she gives the option to trade Bayld for HPB. For every 10,000 Bayld, she'll reward you 1 H-P Bayld in return. She'll also as an incentive provide you with one free HPB for buying 100 HPB in one transaction.
So basically, the most effective convert method for high-bayld players is to trade in multiples of 1million bayld at a time, resulting in 101 HPB.
It's a piss poor conversion rate, but hey, Free HPB is free.