Its been a while, but I do remember PC/XTs and Tandys in the $8k range. Seems like a straight PC listed $2 to $3k. No HD and 128k RAM. Prices came down, with a bump when the 8086 replaced the 8088, and then shot back up when the 80286 took over. (Somewhere I have a rare 80186, it came out of a printer I think)
I got the PC at a Hamfest (search it, along with your city or county, you might get hooked) for $129. Ram, HD, mathco and CGI graphics probably set me back another $500 to $1000, and I wasn't buying COTS stuff at list, components werejust really expensive back then.
I still have that box, and it should still boot. I have been thinking about MIDIing it up again, just for the pure hell of it, but I don't want to devote space to it, and man...using it would be sooo painful now. Cakewalk was running under DOS back then. No mouse support. Just about wore out the arrow and tab keys on the keyboard (which you really couldn't do, the IBM stuff was built like a TANK back then), and had to remember a bunch of text commands and shortcut keys.
I think you are basically right on the audio engine and handoffs to plugins. Until I extend blank measures on a piece, the tails chop at song end, and are there waiting for me at the beginning of the next Play command, back at the start of the piece. With cans on, ( I monitor pretty loud), it can wake you up, even when you expect it!