Used ADI-8 Pros generally sell for $450-$600 if you can find one on eBay. I bought two of them in that price range off eBay several years ago - much cheaper than another FF800 (but you need to have good timing to run across one exactly when you need it).
There are tons of ADAT Line Level AD/DA's out there - you don't have to stick with RME - but the used ADI-8 Pro's are rather nice for the price IMO. Cheaper stuff like an ADA8000 or a Tango-24 might fit the bill - or you might want to stick with higher-end stuff (AD16x, Lynx, ADI-8, another FF800, etc). Your call and certainly your cash...
MADIface-XT+ARC / 3x HDSP MADI / ADI648
2x SSL Alphalink MADI AX
2x Multiface / 2x Digiface /2x ADI8