Atlético bought Caminero from Valladolid when they were in Segunda, dunno what they paid but it's the biggest profile player I can think moving from a Second to a First division club in Spain.
Have in mind that a player not in the top flight will have a bag of crisps as release clause, so big money moves are very unlikely.
Are release clauses a big thing in spain VI? seems like a lot of La Liga players have them in their contracts.