For arguments sake, these rooms are potentially being sublet at the old rate. When they come up for renewal, the rate may jump to the new price (which the OP has said that any of the full apartments up for renewal are going at) of €800p/m.The OP just said that there are rooms available in the same block for EUR 650 p/m. The owners are trying to charge EUR 800.
A (couple of) room(s) at €650p/m isn't enough to establish a market rate. They need to provide as much information as possible if they want to put forward a detailed and robust case.
(It's information which should easily be attained [and which I presume will assist the OPs case hence the (repeated) suggestion to attain it])