There is a going rate for player contracts at each respective position. Generally, positions are valued based on the perception of their importance to team success. The most glaring difference in positional value is of course at quarterback and following far behind are pass rushers and pass protecting tackles.
The draft on the other hand, values player contracts based on the order in which player's are selected, regardless of position. Draft boards aren't simply a line up of players by talent. They are weighted by financial value.
See NFL.com's somewhat outdated rundown on contracts signed by 2016 draftees from the first round. The contract values follow a waterfall pattern.
Naturally, you might expect a correlation between positions drafted at the top of the 1st round and the highest paid positions in football. While a player may be the most talented run-stopping DL in the class, they are less likely to supplant someone who is a bit less adept as a pass rushing 3-4 OLB. Not only due to perceived importance of either specialty, but also due to the monetary implications. See the chart below.
Top 5 Draft Picks by Year
You'll recognize which positions are more sought after at the top of drafts. Clearly teams want the most talented quarterbacks, offensive lineman, and pass rushers within the top 5 selections. At the top is the quarterback position with 9 selections in 6 years. It would be higher too if 2013 wasn't spearheaded by the likes of EJ Manuel, Geno Smith, Mike Glennon, and Matt Barkley. Maybe the weakest QB class in a decade with no proven starters as of yet.
There have been very talented corners, receivers, safeties, and running backs in the last 6 drafts, so why are they so underrepresented in the top 5? Refer again to the rundown of 2016 draftee contracts. As an example, the estimated value for the Bears' pick at #3 this year is $27 mil over 4 years and of course, a 5th year option. With that in mind, check out overthecap.com's rundown on quarterback pay versus the rundown on safety pay. Now imagine the Bears select Jamal Adams, a fantastically talented safety out of LSU. At $27 mil, he would be making slightly above market average for a proven NFL starting safety. He is probably worth that, as talented as he is, but he definitely doesn't come at a cost savings. Now, a QB on the other hand, whomever the Bears would maybe select at #3, making $27 mil, would be over $50 mil below market average for a proven starter.
By saving money on a talented player at a high value position, you can now purchase players in free agency for values that you would likely have paid for an early draft pick at the same position anyway.
This is why organizations take risks on guys at pivotal positions. If by any chance that QB plays at a high level, you have 5 years and a savings that will only increase year after year. So if there is even an iota of belief that a QB or a pass rusher could have moldable talent, do not be surprised if their names are amongst the very first announced, even leapfrogging players that are more capable at their own craft. Keep this in mind when adjusting your first round mock draft.