There are inexpensive ($10ish) converters that do DP > HDMI, but the inverse is much more expensive ($50-100)

That's because DP sources can (and nearly always do) support encoding HDMI as a secondary mode, so all you need is a passive adapter. Going the other way requires active conversion.

I assume you have to pay HDMI royalties for DP ports which support the full HDMI spec, but older HDMI versions were supersets of DVI, so you can encode a basic HDMI compatible signal without stepping on their IP.

As long as the port supports it passively (called "DP++ Dual Mode"), if you have a DP-only port then you need an active converter which are the same as the latter pricing you mentioned.