Transavia awards are now bookable online. Prices vary from a little cheaper, to identical to KLM awards. Taxes are pretty much identical too.
No sure why Transavia thinks they will sell any awards for AMS-ATH at 15K miles plus tax, while regular prices are about 70 euros one way. It is bad enough that KLM dares to ask such high prices, but Transavia...one would have to be crazy to pay 15K and redeem on Transavia.