They aren't rare at all. All rich/sporty vehicles, when having a randomised numberplate assigned, have a 1 in 3 chance (pretty common) of a vanity plate:
new SportyPlates[][] =
{
"V1S10NRY", "D0M1N0", "DR1FT3R", "5MOOTH", "5T34LTH", "BE4STY", "P0W3RFUL", "R4C3R", "N0F00L", "TOPCLOWN",
"B055", "B4N4N4", "TROP1CAL", "DE4DLY", "E4TM3", "TH370S", "B0UNC3", "GUNZOUT", "0R1G1N4L", "CH4RG3D",
"CRU151N", "MU5CL3", "IN4H4ZE", "TPD4WG", "B1GC4T", "BUZZ3D", "M1DN1GHT", "W1P30UT", "T0FF33", "IML4TE",
"D1RTY", "0BEYM3", "4LLOY5", "FRU1TY", "SP33DY", "F1D3L1TY", "5H0W0FF", "1MSORAD", "31GHT135", "CURV35",
"PR3C10US", "H3R0", "W1NN1NG", "M1DLIFE", "R3G4L", "SL1CK", "B1GB0Y", "PR3TTY", "C4TCHM3", "HOT4U",
"J0K3R", "V1P", "OH3LL0", "PH4R40H", "SL33K", "D34TH4U", "2FA5T4U", "L4WL3SS", "D3V1L", "T0PSP33D",
"B35TL4P", "M4K3B4NK", "FUTUR3", "B1GMON3Y"
};