While I have no inside info re future upgrade costs or timing (and would be under NDA if I did), I can offer an observation and an opinion that may be of help.
First, as to when FM14 is likely to hit: From FileMaker 7 through, roughly, FileMaker 11, they seemed to be on a 15-18 month cycle. The last couple of versions, however, have been closer to 2 years apart. As FileMaker 13 was released last December, that precedent puts FileMaker 14 at least half a year out and more likely closer to a year away.
Second, I think FileMaker 13 has so many improvements over 12—custom themes and styles, PSoS, encryption at rest, to name a few—that the upgrade cost is absolutely worth the gains in productivity, performance, security, etc., that you'll be able to enjoy between now and the eventual release of 14.
P.S. OTOH, on rereading your post, I note that you said you're not doing actual development at this time, so my advice may not be entirely applicable to you. Hopefully helpful, nonetheless, in some small way.
If you are learning it in your spare time, it is worth upgrading so that you'll have a chance to review the new features of FileMaker before any newer versions come out. As the above post stated, there is a lot of new exciting and game-changing features in FM 13. If you don't have those down going to the next version will be a bigger jump.
It comes down to your willingness to invest money in your learning if you do decide to come onboard the FM development bus.