Yup, by its nature surplus stuff comes and goes. And sometimes comes and goes again.
The trick is when you see a killer deal like $120 tins of surplus 5.45, or $10 surplus 5.45 mags, you just assume that it won't last forever and buy as much as you can reasonably afford. After hearing everyone talk about "the good ol' days" of $99 Wolf 7.62x29 cases, I decided I wasn't going to miss the boat on 5.45, and bought enough to equip the entire Peruvian Army.
My guess would be that we'll see surplus 5.45 again, but not at the old prices. Still, I think that anything under $200/tin is a good deal. Not nearly as good as it was, but a good deal nonetheless. With mil-spec 5.56 running at least $300/case, the 5.45 at $200/case would still be comparatively cheap. And while the Hornaday ammo is not cheap, it's not insanely expensive either. A case of Hornaday 5.45 will run you about $370; roughly the same price as a case of M855/SS109 5.56.
I still think 5.45 is worth getting into. It's just not as cheap as it's been in years past. Then again, the same could be said for a lot of rifles.