There are a couple of after market suspension outfits "re-building" the ESA units for less than that, for the GS anyway. What they're doing is fitting the ESA plumbing and stuff to their own shocks, with or without spring upgrades giving you a completely custom tuned set up that can be rebuilt in future. After one season with ESA, I'd never buy another ST or similar bike without it.
Yeah, Works, HyperPro and Wilbers has ESA compatible shocks... they are about the same as the price of non-ESA shocks... I looked into them when my original shocks needed replacing at 42K... I opted for Ohlins instead and am happy with that decision. But you can retain the ESA functionality with aftermarket and save a lot over the BMW ones. BTW, rumor has it that Ohlins are working on ESA compatible replacement shocks as well.