Pro Football Focus has a 
free agent tracker which includes each player's 2018 PFF grade and number of snaps played. 
Pro Football Logic also has player ratings.
I decided to throw together a formula using each of those numbers to estimate how much each FA should be worth. I made 2 estimates for each player, one based on his 2018 PFF grade and one based on his Pro Football Logic rating. On the whole these estimates line up reasonably well with the FA contracts that we've seen.
Here are the 25 most overpaid players among this year's free agents in amount per year, where I credited each player with whichever estimate was more optimistic (so these are the guys that both PFF and PFLogic were down on):
Salary    Pos     Player           Team
	$13.5M/yr    LB    Kwon Alexander    SF
	$22M/yr    QB    Nick Foles    JAC
	$16.5M/yr    EDGE    Za'Darius Smith    GB
	$14M/yr    S    Landon Collins    WAS
	$13.8M/yr    T    Donovan Smith    TB
	$12M/yr    EDGE    Dante Fowler Jr.    LAR
	$7M/yr    T    Greg Robinson    CLE
	$12.3M/yr    DI    Sheldon Richardson    CLE
	$11.1M/yr    C    Mitch Morse    BUF
	$16.5M/yr    T    Trent Brown    OAK
	$7.3M/yr    G    Ty Nsekhe    BUF
	$13.1M/yr    HB    Le'Veon Bell    NYJ
	$7M/yr    LB    Preston Brown    CIN
	$6.3M/yr    G    Jamon Brown    ATL
	$7M/yr    G    Billy Turner    GB
	$13.5M/yr    LB    Anthony Barr    MIN
	$4.5M/yr    T    Seantrel Henderson    HOU
	$6.3M/yr    TE    Tyler Kroft    BUF
	$7.5M/yr    CB    Robert Alford    ARI
	$4.5M/yr    DI    Jordan Phillips    BUF
	$11M/yr    WR    Tyrell Williams    OAK
	$4.8M/yr    G    Ty Sambrailo    ATL
	$4M/yr    G    Jon Feliciano    BUF
	$10M/yr    WR    Devin Funchess    IND
	$5.5M/yr    CB    Buster Skrine    CHI
	And here are the 20 most underpaid players in amount per year, where I credited each player with whichever estimate was more pessimistic (so these are the guys that both PFF & PFLogic liked):
	            
	Salary    Pos     Player           Team
	$13.3M/yr    EDGE    Brandon Graham    PHI
	$8.5M/yr    LB    Jordan Hicks    ARI
	$15.4M/yr    EDGE    Jadeveon Clowney    HOU
	$3.5M/yr    S    Ha Ha Clinton-Dix    CHI
	$5.3M/yr    S    Eric Weddle    LAR
	$7.7M/yr    EDGE    Cameron Wake    TEN
	$4.1M/yr    G    Ramon Foster    PIT
	$2.5M/yr    DI    Steve McLendon    NYJ
	$7.7M/yr    T    Bobby Massie    CHI
	$1.6M/yr    EDGE    Brooks Reed    ARI
	$3.8M/yr    EDGE    Lorenzo Alexander    BUF
	$11M/yr    CB    Kareem Jackson    DEN
	$6M/yr    TE    Nick Boyle    BAL
	$9M/yr    S    Adrian Amos    GB
	$8.4M/yr    DI    Henry Anderson    NYJ
	$14M/yr    CB    Tyrann Mathieu    KC
	$5.3M/yr    LB    Thomas Davis    LAC
	$4.5M/yr    WR    Danny Amendola    DET
	$15.2M/yr    DI    Grady Jarrett    ATL
	$2M/yr    HB    Frank Gore    BUF
I have included franchise tagged players here. The lists leave out RFAs and guys who were traded and then signed a new contract, and maybe some others.
Obviously NFL teams know some things that aren't incorporated into PFF or PFLogic ratings, and my method for turning those ratings into salary estimates is imperfect, and the thing I'm doing here will tend to underrate guys like Bell who missed time in 2018. But these at least seem reasonably plausible as lists of good & bad signings.