if you look at history a lot of inventions have been made by accident or began at one person and were from that point on improved. the basic idea is that you should not pay to research new technologies as if you had to pay for them but rather have your people discover them incidentally, the great inventions we know mostly began with 1 person and not the money of a whole nation, after which they were enhanced by either the same person or researchers. in my opinion techs should be discovered incidentally and be enhanced (through financing research) after being discovered. the chance that new techs will be discovered should be enhanced by the size of your population, education, other support toward the intellectual capabilitys of your people and the presence of the undiscovered tech in neigbhouring civilizations with whom you have good connections such as many trading routes. perhaps there should be several civics that inhibit new discovery's such as theocracy, which in itself is not very tolerant of anything new (in this case, religions). perhaps the chance that a certain will be discovered should be influenced by how much it is needed, as many inventions were made as they were solved the problems that their inventors saw or experienced (for example medicine).