Technology does trickle down the price tiers over the years, but it takes a few generations for new mid-tier groupsets to overtake the performance of older groupsets from a higher tier - usually it only happens with a "breaking change" - going from 10 to 11 speed, introducing hydraulic disk brakes as a groupset option, etc. Also keep in mind that each generation stays current for 3-4 seasons before being replaced.
Given that, it is possible that some of the cheaper bikes may be using the last generation of 105, but as long as it is only one generation back you are still getting better components than current year Tiagra. Two generations back will be roughly comparable. At three or more, you're probably better off with new Tiagra, but at that point you're looking at decade old components - no new bike will come with such old componentry.
Edit: I just found this full history of Dura-Ace releases. While it's obviously representative of Shimano's latest and greatest and doesn't directly reflect how quickly that new tech trickles down, I think it still gives a good sense for how quickly change does, and doesn't, progress in groupset technology.