Hopefully i can help.
Its not an uncommon fault with these causing problems like you have on Renaults that use this sort of entry/starting system and its usually or the most common either the card at fault or the reader so its a throw up between the two if that type of message appears where the card isn't getting detected - one way to try and help determine wether it is actually the reader or the card for example is to try another card if you have one - if works allot better or the problem doesn't happen to second/spare one then you can pretty much assume the card your using before is the problem and not really down to the reader and they can be repaired http://www.keyrepair.co.uk/index.html -
Be aware secondhand cards from the likes of ebay won't work as to replace these cards you need to order from Renault only - if it happens to both cards then its most likely the reader or in some cases a more in depth problem apart from the reader itself which will need a Renault clip diagnostic machine to track that down properly sadly usually Renault would need to track that type of fault down unless you have good independent garage that has that machine or mobile diagnostic technician -
All i can suggest is try and maybe change the batteries in the key you have if you havent done so already - try and clear out the reader slot ie:- basically try and hoover out the slot as best you can - as its possible for a build up of dust etc to get trapped inside over time and causing the little plastic arms/microswitch not be activated properly when the card is pushed in - or maybe try and build up the thickness of the key card up.. for example draw round the card with a very thin bit of card with an over lapor tab so it doesn't get stuck in reader then insert the key card & thin card along with it to make it a tighter fit into the slot - if that works alot or a little better then you can pretty much assume the reader is at fault or needs taken out and checked further or stripped down and cleaned a little better and or in most usual cases if the reader is at fault another may have to be installed these don't really need to be coded in if another is to be installed it usually just a case of aquiring another reader for your type of car and plug it in place of the old one / obviously to this you will need to remove parts of the dash trim to get the reader assembly out..
It would be a little difficult to put a specific price on that particular job depending of course on the garage you use charges for labour per hour as all garage are differant across the uk - so if the reader is suspect then maybe try looking out for a second hand one maybe from a breakers as above or on ebay as some do seem to turn up on there quite often or wether it just needs removed and cleaned up a bit - that can be done and maybe something to have you garage try first at least to see if thats all it needs and to see wether anythings broken inside the reader - you could also try the hoover out method just incase thats all it needs and may try the building up the card method to see if that narrows down the fault..I do hope this helps..