The "bang for the buck" is better for later round QBs only when you find one worth a darn. The Tom Bradys and Kurt Warners are few and far between. You can draft mid-late round QBs for decades without hitting on one thats a keeper (or at least a starter). Consider how many QBs have been drafted in the mid to late rounds over the past 6 or 7 years that simply disappeared into obscurity. Yes, the risks and certainly the contract costs are higher with a 1st round guy, but so is the potential for success.
If you are in need of a QB, and there are QBs available that are considered to be 1st round material, you gotta pull the trigger.
Nothing ventured, nothing gained.