I patiently collected a rival group piece by piece from ebay last winter. Mostly new but a couple of take off items. Best score was a take off crank with a brand new BB for $41. The seller had upgraded to force and left the original bb in. Also got take off derailleurs (FD for $10) but everything else was new Rival except a 6600 cassette. Total cost was $440, with shipping probably around $500. I'm sure you could do as well with 105. Interestingly, I wasn't against Apex parts where they would work but they were never cheaper. Maybe because it was still a fairly new group. So I guess I wouldn't be suprised to find 105 at better prices than 10sp Tiagra.

And listen to the posters about monitoring Bonktown. They have had more groupset components pop up in the last 2 months than I have seen in the 2 years before that.