How can I obtain the raw decimal value of an RSA key's modulus?
You've downloaded Google's (or whomever's) certificate. You have it stored in a file called GoogleCert:

$ cat GoogleCert
-----BEGIN CERTIFICATE-----
MIIIdDCCB1ygAwIBAgIIWOU4T7CaLpQwDQYJKoZIhvcNAQELBQAwVDELMAkGA1UE
BhMCVVMxHjAcBgNVBAoTFUdvb2dsZSBUcnVzdCBTZXJ2aWNlczElMCMGA1UEAxMc
...
JmztPlaPUkFUEnAL2aWCAQCHscxb9bMFDmUW/kv/UnBNBBX7PRvl5rPjPeFk9yS5
HU2vHiQa8VY=
-----END CERTIFICATE-----


Use the command openssl x509 -in FILENAME -noout -modulus argument to extract just the Modulus:

$ openssl x509 -in GoogleCert -noout -modulus
Modulus=AAC42564448507CB736A37F...9152FD0458ECF42145


Copy and paste the HEX output into BC using the ibase=16 argument to indicate the input is in Hex:

$ echo 'ibase=16;AAC42564448507CB736A37F...9152FD0458ECF42145' | bc
21557235071938158553153701887793754486919886530446620906710060142975\
...
80517225349695620402252397302295002436481842156355298655378997632358\
23941