Indeed, with a promo ticket having about €98 in taxes (assuming the return routing is a similar cash amount! I'm not going back to check!!) then you are almost paying out as much cash as you would be for a cheap revenue ticket (booked early enough, with a starting ~€99-ish pricepoint, you could just buy a Lufthansa or KLM revenue ticket (well, these days that route might be staring at €110 or €120 Euro)).
It just confirms how awful promo awards are for short haul European redemptions (in comparison to, say, BA redemptions in Europe - though these clearly are best enjoyed if you are based at LHR, or at least in the UK (the connecting flight to LHR is "free") A FRA-LHR-FRA return on BA costs just 9,000 Avios and €42 and those Avios are far more quickly earned than the 10,000 FB miles you need for this special almost-unique 50% discount!!!).