Perhaps the easiest way to value the rookie picks is by ADP. It is just up to you if you agree with the ADP or not.
I found t
his website which is free to sign up and they have some recent ADP data that includes the rookie picks. It seems like this website focuses on this a lot.
Anyhow here is their ADP for the rookie players.
Name Position Age ADP (ADP trend data)
Leonard Fournette RB 22 21.96 +3.3 6409
Dalvin Cook RB 21 25.09 -4.4 5995
Corey Davis WR 22 28.04 +2 5485
Mike Williams WR 22 43.91 -7.6 4017
Christian McCaffrey RB 20 44.41 +19.9 3929
Joe Mixon RB 20 48.16 +11.8 3758
JuJu Smith-Schuster WR 20 69.21 -6.9 2355
John Ross WR 21 71.63 -1.6 2155
OJ Howard TE 22 85.71 +9.5 1725
Alvin Kamara RB 21 91.75 +12.8 1509
Evan Engram TE 22 105.33 +21.7 1034
D'Onta Foreman RB 21 109.29 -1 905
Samaje Perine RB 21 111.58 -16.9 866
David Njoku TE 20 114.33 +2.4 828
Chris Godwin WR 21 118.13 +63.7 757
Carlos Henderson WR 22 123.75 -2.8 678
Kareem Hunt RB 21 130.34 -8.3 606
Curtis Samuel WR 20 132.38 -1.4 555
Zay Jones WR 22 133.63 -0.6 542
Cooper Kupp WR 23 148.29 +6.4 389
KD Cannon WR 21 149.21 -33.2 380
Jamaal Williams RB 22 160.79 -14.1 304
Jeremy McNichols RB 21 167.38 +10.3 278
James Conner RB 21 175.54 +15.2 255
Malachi Dupre WR 21 175.75 -38.9 249
Dede Westbrook WR 23 180.59 -28.8 228
Bucky Hodges TE 21 184 +22.8 186
Wayne Gallman RB 22 186.84 -39 174
Taywan Taylor WR 22 188.84 -4.5 163
Deshaun Watson QB 21 196.13 -0.8 156
Isaiah Ford WR 21 199.16 -48.9 140
Elijah Hood RB 21 209.66 -12 114
ArDarius Stewart WR 23 212.09 -9.8 102
Patrick Mahomes QB 21 214.63 +21.4 94
Jake Butt TE 214.79 -23.6 92
Josh Reynolds WR 22 217.75 +10.9 86
Marlon Mack RB 221 0 75
Elijah McGuire RB 22 221.75 -26.6 72
Mitch Trubisky QB 227.75 +3.8 61
Chad Hansen WR 22 232.04 -37 55
DeShone Kizer QB 21 233.91 -6.6 46
Corey Clement RB 22 238.63 -47.1 31
Amara Darboh WR 23 239.59 -11.8 27
Noah Brown WR 239.75 -29.6 26
Donnel Pumphrey RB 240 0 25
Jordan Leggett TE 240.75 -2.9 23
Kieren Duncan WR 23 300 0 2
Marquis Bundy WR 22 300 0 2
Marcus Leak WR 24 300 0 2
Chris Hubert WR 23 300 0 2
Ed Eagan WR 23 300 0 2
Mekale McKay WR 23 300 0 2
Joel Stave QB 24 300 0 2
Chris Swain RB 24 300 0 2
Jameill Showers QB 25 300 0 2
Quinshad Davis WR 22 300 0 2
Jordan Williams WR 22 300 0 2
CJ Ham RB 23 300 0 2
Darius Powe WR 23 300 0 2
Jamel Johnson WR 25 300 0 2
Mose Frazier WR 23 300 0 2
Rashaun Simonise WR 21 300 0 2
Ben Roberts WR 24 300 0 2
Brandon Ross RB 24 300 0 2
Cedric O'Neal RB 23 300 0 2
Max McCaffrey WR 22 300 0 2
Tevin Jones WR 24 300 0 2
Alex Ellis TE 24 300 0 2
Hunter Sharp WR 22 300 0 2
Darion Griswold TE 24 300 0 2
Jamal Robinson WR 24 300 0 2
Andy Jones WR 22 300 0 2
Mitch Mathews WR 26 300 0 2
Beau Sandland TE 24 300 0 2
Anthony Dable WR 28 300 0 2
Steven Scheu TE 24 300 0 2
Kyle Carter TE 24 300 0 2
Jake McGee TE 25 300 0 2
Bryce Williams TE 24 300 0 2
Chris Brown WR 23 300 0 2
Artavis Scott WR 22 300 0 2
Blake Frohnapfel QB 300 0 2
Shelton Gibson WR 23 300 0 2
Travin Dural WR 23 300 0 2
Chad Kelly QB 300 0 2
Terron Beckham RB 300 0 2
Ryan Switzer WR 300 0 2
Speedy Noil WR 300 0 2
Jay Lee WR 23 300 0 2
Cayleb Jones WR 24 300 0 2
The 1st overall pick is being valued as ADP 22 and so on from there. So you would just cross reference the ADP to whatever ranking you are using.