Actually, I'm pretty sure that best bang for your buck by FAR is the lowest-entry level group (likely Shimano 2200 or 2300).

No, it's definitely NOT as good as 105-DA and even a new road cyclist will notice the difference. But will it shift reliably and accurately for everything except rapid hi-pressure (think criterium attacks) shifts? Yes.

You might 'think' you NEED 105 (or Dura-Ace) but in reality, you'd be able to ride perfectly fine on a 2200 group, even if you're a very strong rider. And no, they don't wear out crazy fast either - my entry level Giant Defy3 is a Sora/2200 mix triple and still shifts fine even after 3.5 years, with the only replaced in that time being the chain.

Shimano even doesn't make the 2200 or Sora groups easy to purchase unless you're buying a whole entry level bike - your LBS will almost certainly not carry a retail-packaged set of those, whereas they'd happily and readily sell you 105 and up groups. (They could probably do a 'deal' with OEM parts.)