When I first offered LED conversion kits, I got the inverters I modded for free from forum members who sent their spares, supplemented by bulk lot buys of used boards from eBay. For the first 200 kits or so, I paid on average about $5 a board, which I then modded with the custom LED hardware.
The supply of X60 and X61 inverters is drying up, which is not to say they're no longer available, but they're now well into legacy pricing. Min price is about $20 apeice now.
Building complete inverters from scratch was probably always cost effective, but I just couldn't find the discontinued connectors I needed. I think I have those secured now, so I spent a few days of free time consing up a new inverter board design.
The prototype is off to OSHPark for fabbing! It's nearly the same schematic as the TLD3, but the layout is from-scratch to make it easier to assemble.