There is no doubt that a low £/€ exchange rate will deter UK buyers and also the uncertainty at home regarding the economy and the state of play in Spain relating to the British becoming resident in Spain. One can only assume that the current rules applying to non EU people will be a starting point. whether or not Spain will make special rules for us will depend perhaps upon Gibralter and any deal the UK government may do. But that is down the line as it is hardly a priority
Existing owners are unlikely to be affected except in relation to taxation as we will be treated as Non EU citizens!
So taking UK buyers out of the market will reduce the possible increase in values which was starting to happen but other demand from Northern Europeans which has from all reports been strong recently, will be unaffected
So Brexit must be negative for property values on the Costas but a lot depends upon the Spanish economy which is showing signs of growth and this may stimulate local demand which counter balances the Brit situation
Any one selling may benefit from the low exchange rate- many bought at 1.50+ and taking it back at 1.23 or below is a bit of a buffer in £ terms and enables Brits in Spain being able to accept a lower € price
With all these issues it will be interesting to see what happens! But the continuing increase in values in Brit locations which was starting will I suspect now go back to previous levels